System and method for evaluating the performance of a scan strategy

ABSTRACT

A system and method is provided for detecting emitter signals and for determining a scan strategy for a receiver system that receives such emitter signals. Once a scan strategy is computed, it may be desirable to evaluate the scan strategy&#39;s intercept performance against a given emitter set under specific altitude, range and receiver load conditions. 2D and 3D emitter scan patterns are modeled and simulated. The receiver scan is also modeled and simulated. Performance statistics from the simulation are collected and analyzed.

RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) tocommonly-owned U.S. provisional patent application Ser. No. 60/427,103,entitled SYSTEM AND METHOD FOR SCAN TABLE ANALYSIS AND GENERATION, filedon Nov. 18, 2002, which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

The invention generally relates to signal detection, and moreparticularly, to detecting and analyzing signals generated by emitters.

BACKGROUND OF THE INVENTION

Detection systems exist for detecting signals generated by emitterswhich are of interest. For example, there are scanners (e.g., a policescanner) that are capable of scanning a frequency band for transmissionswithin that frequency band. In the case of a police scanner, channelsare scanned sequentially to find a signal of interest. Scanning isachieved by tuning receiver hardware to a particular frequency toobserve one or more transmissions within that particular frequency.

There are more sophisticated systems to detect transmitted signals thatuse other methods for determining signals of interest. For instance,there are what are referred to as Electronic Support Measures/ElectronicIntelligence (ESM/ELINT) systems for conducting surveillance (e.g.,radar, and other signals across a wide range of frequency spectrums).These systems detect one or more signals produced by emitters (oftencalled “threats”) that are detected and observed.

For example, in a military aircraft or other vehicle, enemy signals(e.g., radar) may be observed that are capable of detecting the vehicle(e.g., an airplane). These threats may need to be determined prior todetection to ensure the safety of the vehicle, and are often observedand classified to identify the particular threat. For example, certainsignals may have particular signatures that are indicative of certaintypes of emitters. Further, there may be a need to detect and identifythe location of a threat (e.g., a radar installation) for targetingpurposes.

There is a problem in that there may be multiple threats but only afinite number of resources to detect them. More particularly, there maybe hundreds of threats, but receiver capabilities do not allow allthreats to be observed simultaneously at all frequencies. However, thereis a need to scan the frequency spectrum in an efficient manner todetect all of the signals of interest. In some cases, there is a need tohave assurance that a threat will be detected in time to respond to thatthreat. In the case of detection of a radar emitter by a vehicle, it maybe also necessary to detect the threat before the threat is capable ofdetecting the vehicle.

There is difficulty in balancing the need for detecting each of numerouspossible threats because of the finite resources of the detectionsystem. That is, hardware and/or software (e.g., memory, processingcapability, etc.) of the detection system may be limited to monitor onlycertain portions of the frequency spectrum of interest or may be limitedto detecting a limited number of threats. Practically, there are anumber of threats that are concurrently transmitting that should bedetected, but it is expensive from a hardware standpoint to monitor allfrequencies of interest at all times to detect all threatssimultaneously. For example, U.S. Pat. No. 6,020,842 discloses onemethod for improving the probability of intercepting data transmitted ina number of different frequency bands. In summary, there is a continuingneed for improved methods for detecting and analyzing emitter signals.

SUMMARY OF THE INVENTION

According to one aspect of the invention a method of detecting anemitter signal is provided. The method comprises acts of: a) receiving ascan strategy to detect at least one emitter, the scan strategycomprising a plurality of dwells; b) simulating a scan of an emitter;and c) evaluating the performance of at least one of the plurality ofdwells of the scan strategy based on the scan of the emitter.

According to another aspect of the invention, a computer-readable mediumis provided, the computer-readable medium having encoded thereoncomputer instructions which when executed by a computer system cause thecomputer system to perform a method comprising acts of: a) receiving ascan strategy to detect at least one emitter, the can strategycomprising a plurality of dwells; b) simulating a scan of an emitter;and c) evaluating the performance of at least one of the plurality ofdwells of the scan strategy based on the scan of the emitter.

According to yet another aspect of the invention, a receiver system fordetecting an emitter signal is provided. The receiver system comprises amemory having stored therein a scan strategy, the scan strategy havingbeen evaluated by a system that performs acts of: a) receiving a scanstrategy to detect at least one emitter, the scan strategy comprising aplurality of dwells; b) simulating a scan of an emitter; and c)evaluating the performance of at least one of the plurality of dwells ofthe scan strategy based on the scan of the emitter.

Further features and advantages of the present invention as well as thestructure and operation of various embodiments of the present inventionare described in detail below with reference to the accompanyingdrawings. In the drawings, like reference numerals indicate like orfunctionally similar elements. Additionally, the left-most one or twodigits of a reference numeral identifies the drawing in which thereference numeral first appears.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description when taken inconjunction with the accompanying drawings in which similar referencenumbers indicate the same or similar elements.

FIG. 1 is a block diagram of a detection system according to oneembodiment of the invention;

FIG. 2 is a block diagram of a detection system according to anotherembodiment of the invention;

FIG. 3 is a flow diagram of a process for determining scan strategyaccording to one embodiment of the invention;

FIG. 4 is a block diagram of an emitter database according to oneembodiment of the invention;

FIG. 5 is a flow chart of a process for evaluating an antenna modelaccording to one embodiment of the invention;

FIG. 6 is a flow chart of another process for evaluating an antennamodel according to one embodiment of the invention;

FIG. 7 is a chart showing an example emitter signal that can be receivedand detected according to one embodiment of the invention;

FIGS. 8A-8B are charts showing how signals are measured without and withpulse grouping logic, respectively;

FIG. 9 is an example showing how emitters having different sets of rangecontrols may be satisfied by selecting dwells according to oneembodiment of the invention;

FIGS. 10A-10B are charts showing time in beam (TIB) extraction fordifferent sensitivity levels according to one embodiment of theinvention;

FIG. 11 is a diagram illustrating the placement of dwells in relation tothe pulse repetition intervals of emitters, according to one embodimentof the invention;

FIG. 12 is a diagram illustrating two possible dwell placement solutionsfor detecting an emitter, according to one embodiment of the invention;

FIG. 13 is a table showing an example of a portion of an informationmatrix, according to one embodiment of the invention;

FIGS. 14A and 14B are diagrams showing possible dwell placementsolutions for the information matrix of FIG. 13, according to oneembodiment of the invention;

FIG. 15 is a flow chart illustrating an example of a method for creatinga scan strategy, according to one embodiment of the invention;

FIG. 16 is a flow chart illustrating an example of a method for creatinga scan strategy, according to one embodiment of the invention;

FIG. 17 is a diagram illustrating two possible scan strategies generatedby using different initial limit values, according to one embodiment ofthe invention.

FIG. 18 is a flow chart illustrating an example of a method for creatinga scan strategy, according to one embodiment of the invention;

FIG. 19 is a table showing emitter timing data, according to oneembodiment of the invention;

FIG. 20 is a table showing emitter timing data and dwell cost, accordingto one embodiment of the invention;

FIG. 21 is a diagram showing the timing of execution of dwells,according to one embodiment of the invention;

FIG. 22 is a flow chart illustrating a method of selecting a non-maximumdwell duration, according to one embodiment of the invention;

FIG. 23 is a flow chart illustrating a method of

FIGS. 24A-24F are examples Solution and Data matrices, according to oneembodiment of the invention;

FIG. 25 is a diagram illustrating dwell coverage of emitters afterrounding down dwell minimum frequencies, according to one embodiment ofthe invention;

FIG. 26 is a diagram illustrating dwell coverage of emitters afterrounding down dwell minimum frequencies, according to one embodiment ofthe invention;

FIG. 27 is a flow chart illustrating a method of compensating for tuningstep coverage gaps, according to one embodiment of the invention;

FIG. 28 is a flow chart illustrating a method for verifying andallocating receiver system resources, according to one embodiment of theinvention;

FIG. 29 is a flow chart illustrating a method for verifying andallocating receiver system resources, according to one embodiment of theinvention;

FIG. 30 is a diagram showing an emitter's frequency range broken up intodiscrete pieces, according to one embodiment of the invention;

FIG. 31 is a diagram showing simulated detection of an emitter's scanpattern according to one embodiment of the invention; and

FIG. 32 is a diagram showing illumination periods of an emitter's scanpattern, minus integration time, according to one embodiment of theinvention.

DETAILED DESCRIPTION

According to one aspect of the present invention, an improved method isprovided for detecting signals generated by one or more emitters. FIG. 1shows an embodiment in which various aspects of the invention may beemployed. In particular, a method for determining a scan strategy may beemployed in conjunction with a detection system 101 that detects one ormore signals 105A, 105B transmitted by one or more emitters 104A, 104B.As discussed above, these emitters 104A, 104B may be transmitted by oneor more threats 106A, 106B, respectively.

Detection system 101 may include one or more sensors 102 and one or moreprocessing systems 103 that receive and process signals 105A, 105Breceived by sensor 102. These signals 105A, 105B may be, for example,electromagnetic signals transmitted in any one of a number offrequencies, including radar, communication, and other types of signals.In one embodiment, the receiver or receivers may be capable of detectingone or several instantaneous frequency (IF) bandwidth and videobandwidth (VBW) combinations with varying sensitivity. Further, thedetection system may employ alternate, single or multiple methods ofsignal detection.

Signals received from sensor 102 are passed to receive processor 103. Inone aspect of the present invention, receiver processor 103 receives andprocesses multiple signals from one or more sensors 102 and implements ascan strategy. In particular, processor 103 may be capable of detectingone or more threats 106A-B by observing frequency bands defined by thescan strategy. This scan strategy may be provided by one or more systems108A-108B, for example.

As discussed above, detection system 101 may be implemented in variousways. For example, a scan strategy may be computed offline by system108B. The scan strategy may then be transferred to detection system 101,which is mounted in a vehicle. Detection system 101 may then implementthe scan strategy computed by system 108B. Vehicle 107 may be, forexample, an aircraft that flies a particular mission. Although system101 may be installed on or used in conjunction with a vehicle 107, itshould be appreciated that the invention is not limited to being usedwith a vehicle. For example, system 101 may be used as a standalonesystem, or may be a stationary or mobile unit.

System 101 may be implemented in hardware, software, or a combinationthereof. In one embodiment of the invention, various components ofsystem 101 may be implemented in a software program executing in ageneral purpose computer system (e.g., a personal computer (PC)). Itshould be appreciated that the present invention is not limited to anyparticular combination of hardware or software, but rather, the systemmay be implemented with any number and combination of hardware and/orsoftware systems.

System 108A-108B may determine an optimum strategy for scanning themultiple signals according to various embodiments of the invention. Inone embodiment of the invention, processing systems 108A-108B provide anevaluation of the strategy to determine whether each signal of interestis detected within defined constraints.

A determination of scan information (e.g., a scan database in the formof a scan table) that describes an optimum scan strategy may beperformed by system 108A (e.g., system 108A may form part of detectionsystem 101) or by a system 108B external to detection system 101 (e.g.,a computer system configured to determine scan table information fordetection system 101). Any system, either part of or separate fromdetection system 101 may be used to determine a scan strategy. Accordingto one aspect of the present invention, a software program that executeson a PC may accept a number of parameters describing emitters ofinterest, system configuration information and in, one embodiment,actual emitter data to determine an optimum scan table for the detectionsystem 101. The software program may be capable of producing scaninformation in a format that can be used to execute the determined scanstrategy by detection system 101. For a detection system 101 implementedin a vehicle 107 (e.g. an aircraft), the software program may beoperated prior to a mission to determine an optimum scan table, and thescan table may be input to the detection system 101 for use during amission.

FIG. 2 shows a system 201 for determining a scan strategy according toone embodiment of the invention. System 201 is similar in function tosystems 108A-108B described above and is configured to determine a scanstrategy for one or more detection systems (e.g., detection system 215).System 215, similar in function to systems described above, isconfigured to receive one or more signals 208 to be detected andidentified. These signals may be received by one or morereceiver/processor 206, outputs of which are processed by a processor204. Processor 204 analyses and identifies these signals based on thescan strategy and other information provided by system 201 andinformation identified in the received signals.

As discussed above, these signals may be transmitted by one or morethreats, and system 201 may be used in conjunction with a detectionsystem (e.g., system 215) to identify these threats to a user. That is,system 101 may be operated to provide a scan strategy to detectionsystem 215.

Further, system 201 may have an associated interface 203 to receiveinput from and display information to user 202. Interface 203 may be,for example, a graphical user interface of a computer system. User 202may also provide input to system 201 to configure system 201, inputdefine constraints, provide information regarding emitters, or provideany other type of input. According to one embodiment of the invention, anumber of emitters are defined to the detection system 201 by user 202,and based on these emitters, a scan strategy 211 may be determined forone or more of these emitters. As discussed above, a system fordetermining scan strategy may be incorporated in a detection system, ormay be implemented separately, and it should be appreciated that theinvention is not limited to any particular implementation.

System 201 may include an associated storage 205 for storing one or moreemitters 210 that includes one or more emitter entries, a scan strategy211, any constraints 212, and rules 216 for processing emitters.Although FIG. 2 shows storage 205 that is part of system 201, it shouldbe appreciated that storage 205 may be separate from system 201.Further, it should be understood that any portion of the data used bysystem 201 may be stored in any location, either on system 201 orelsewhere.

In one embodiment, an emitter entry includes a number of parameters thatdescribe a particular emitter (e.g., frequency range, antenna type,scanning method used, etc.) Constraints 212 may include, for example,constraints of system 201 which may include, for example, processor 204capability, memory limitations, or any other limitations of hardwareand/or software of system 201 that may effect its performance indetecting and identifying signals 208. Constraints 212 may also includethose limitations posed by a user (e.g., limitation to a particularfrequency band of interest, removal of a particular emitter type, orother user limitation). Scan strategy 211 may include a number ofentries that define one or more “dwells” used to scan one or more of theemitters defined in emitter database 210. A dwell generally defines scanresource solution(s) (e.g., frequency range to be scanned, sensitivity,scan period, revisit time, etc.) that meet one or more emitterrequirements. A dwell may be used to configure a receiver that detectsthe one or more emitters that the dwell is designed to detect.

Optionally, system 201 may be adapted to determine a scan strategytaking into account the operation of one or more jammers 207 thatactively provide signals 209 to a jammer transmitter 214 that “jams” oneor more emitters. That is, system 215 may be capable of producing asignal that interferes with detection by a threat. According to oneaspect of the invention, it is realized that a jammer may have an effecton detection of one or more signals by system 215, and therefore it maybe beneficial to consider activity of a jammer in determining a scanstrategy by system 201.

Determining Scan Strategy

According to one aspect of the present invention, a receiver scanstrategy may be generated that provides optimal or near optimalintercept performance against an arbitrary selected set of emitterparameters. It is realized that a scan strategy that can enableefficiently scanning of a set of emitters and meet the interceptrequirements of each emitter in the set. Also, based on one or moreemitters of the set, a scan strategy may be determined that minimizesthe number of tuning dwells used to detect the emitter(s). In anotheraspect of the present invention, the scan strategy minimizes the numberof receiver resources necessary to detect one or more emitters ofinterest.

FIG. 3 shows a process 300 for determining a scan strategy. At block301, process 300 begins. At block 302, a solution is determined for oneor more emitters of interest. As discussed above, parameters associatedwith each emitter of interest may be input to system 201. System 201 maybe capable of allowing a user to select emitters for which a scanstrategy is to be determined. Optionally, constraints may be defined tosystem 201. A mathematical model may be constructed using emitterparameters and constraints, if any, that can be used to determine tuningdwells and their associated parameters. More particularly, at block 303,a scan strategy may be determined, for example, by determining dwellsthat meet the emitter requirements, selecting an optimum dwell set(e.g., based on cost of executing the dwell set). For instance, dwellsmay be defined that meet the requirements of a set of emitters, anddwells may be compared based on a cost of implementation, and dwells maybe selected as part of a solution set of dwells based on their cost ofimplementation. According to one embodiment of the invention, a databaseof emitter parameters is used to determine a set of receiver tuningdwells that are minimal or near minimal in number and minimal or nearminimal in usage of receiver resources, as allocated in time, the set ofdwells, when executed, allows the receiver to satisfy its interceptrequirements. Optionally, the determined scan strategy may be evaluatedto measure its performance at block 304. At block 305, process 300 ends.

In one embodiment, a database of emitter parameters is created thatincludes intercept requirements for each emitter. In another embodiment,the database may include alternative and/or multiple interceptrequirements per emitter. Emitters in the database or a subset thereofare selected, and dwell parameters are determined for these selectedemitters. According to one embodiment, the dwell solution may beconstrained by one or more solution constraints, if any. In anotherembodiment, inputs are accepted that constrain the solution based onenvironmental knowledge of the operating theater or region. According toone aspect of the invention, each of the emitter model, its dwellsolution(s), constraints, and other related information may be stored inan entity referred to as an emitter database.

FIG. 4 shows an emitter database 401 according to one embodiment of theinvention. More particularly, emitter database 401 includes one or moreemitter entries 403, each of which corresponds to an emitter to bedetected by the detection system (e.g., system 215). Associated witheach emitter entry are a number of emitter parameters 402 which describecharacteristics of each emitter. Emitter parameters 402 may also includeone or more solutions for each emitter (e.g., dwell solutions thatsatisfy the emitter requirements). The emitter database 401 can take theform of an N by M matrix (referred to hereinafter as an “informationmatrix”) that models emitters (N) and their potential solutions (M) inwhich the minimal solution set is contained. As discussed above,constraints may also apply to this N×M information matrix.

Based on the emitter and constraint (if necessary) inputs, dwellparameters may be computed for each selected emitter database entry (Ninputs). This computation may be performed for every possible receivertuning configuration (e.g., IF/VBW combination), yielding severalpotential solutions per emitter database entry (M).

As discussed above, modeling may be used to determine a scan strategy.In particular, a model of the receiver may be employed to establishreceiver characteristics with respect to valid tuning configurations. Anenvironment model may also be included to model electromagneticpropagation between each emitter and the receiver. For example, system201 may model 2-dimensional and 3-dimensional emitter transmitted scanpatterns. These models and their characteristics are then used todetermine hardware settings of a receiver processor for detecting theseemitters within particular constraints as discussed further below.

Determining an Optimal Dwell Solution

As discussed above, the emitter database 401 may include one or moredwell solutions for each emitter. These dwell solutions may include oneor more parameters that specify tuning configurations to detect theemitter signal. According to one embodiment of the invention, emitterdatabase 401 (e.g., an information matrix) may include, for example, oneor more tuning parameters of the dwell including parameters referred toherein as dwell duration and revisit time. Dwell duration is the amountof time spent observing a particular frequency (e.g., a portion of theelectromagnetic frequency spectrum), and the revisit time of aparticular dwell frequency is the time between observations of thatfrequency. Revisit time may be solved for single and multipleillumination time cases, as appropriate.

A ratio of dwell duration and revisit time may be used to approximatethe receiver “cost” of executing the dwell. Given an N by M informationmatrix, a search may be performed for an optimum solution that minimizesthe cost of the dwells, and the solution may be constrained asnecessary. According to one embodiment of the invention, the optimumsolution may be defined as any set of scan strategy dwells that reducesboth the dwell count and receiver usage, the receiver usage beingdefined as the sum of individual dwell durations divided by individualdwell revisit times.

Because a search for the optimal solution using this model is anNP-Complete problem that cannot be solved mathematically, severalmethods may be used to guide, limit and prune the search to avoidsearching exhaustively. These methods include but are not limited toreceiver segmentation (multiple receivers), frequency segmentation,field of view segmentation, pre-filtering and post-filtering techniquesto set and reset potential solution paths, and partial path evaluationto identify and avoid false solutions. It should be appreciated that anymethod may be used to determine an optimal solution, and the inventionis not limited to any particular solution determining method.

According to one embodiment of the invention, the receiver model may beused to take advantage of timing relationships between dwells tominimize the cost function and remove potential scan redundancy. Once anoptimum solution is found, it is still possible that the solution cannotbe realized, given hardware and/or software constraints of the receivingsystem. This condition may be detected and a realizable solution may bedetermined, for example, by fully consuming the constrained capacity,then completing the solution search with the consumed capacity excludedfrom potential solution set. A solution search may be performediteratively on each capacity modeled, until a viable solution is found,or the conclusion is drawn that no viable solution exists (e.g., thereceiving system lacks the hardware or software resources to solve thedesired intercept problem requirements).

Receiver dwells may be outputted by system 201 along with dataindicating estimated real-world performance. This information may beoutput, for example, to a user for evaluation purposes. Scan strategy(e.g., a set of receiver dwells) may be output by system 201 to areceiver system capable of performing the scan strategy. Alternatively,system 201 may be part of the receiver system, and there fore may bepart of a system that executes the scan strategy. The set of dwells usedby the receiver system may be output in a form usable by the receiversystem.

In another embodiment of the invention, it is recognized that ELINT andESM receiver systems are designed to intercept non-cooperative signalsof interest. Because the signals are non-cooperative, the receiversystem analyzes all detected signals present in the environment todiscriminate signals of interest from environmental noise and incidentalbackground signals. Because the detection environment complicatesdetection of emitters of interest, computational and throughput burdensare imposed on the receiver system and these burdens can slow signalintercept performance. Thus, it may be beneficial to reduce the effectsof the processing burden and improve receiver intercept performance inthe presence of significant environmental background energy.

ELINT and ESM receivers employ a scan strategy to scan the frequencyspectrum for signals of interest. This scan strategy comprises a set ofdwells, which define for how long energy is sampled in a portion of thefrequency spectrum, and how often that portion of the frequency spectrumshould be sampled. These are referred to as Dwell Duration (DD) anddwell Revisit Time (RVT) respectively. Dwell Duration may be furthersubdivided into two time intervals, Minimum Dwell Duration (MDT) andExtended Dwell Duration (EDT). A value of MDT defines the shortest timeperiod spent for a given dwell, while a value of EDT defines the maximumamount of time spent for the given dwell. The actual time spent (DwellDuration) ranges between these two limits, based on the signal densityin the sampled portion of the spectrum.

Described below are various aspects of the invention which relate to thedetection of emitter signals and/or determination of a scan strategy.Each aspect, although described below in terms of one or more examples,is independent and therefore each independent is not limited to theexamples, or to any other aspects described herein.

Discontinuity Correction

As discussed above, emitters may be modeled using parameters thatdescribe the transmission characteristics of the emitter signal. Thesecharacteristics are commonly referred to in the art as an antenna model.These characteristics may include, for example, beamwidth, frequencyrange, gain (e.g., in main beam and side lobes), etc.

Often, there may be errors in the models that may affect how theemitters are processed and detected. When revisit times are computed foreach receiver detecting method (e.g., HW bandwidths) for an emitter inan information matrix of emitters, they may not be monotonicallyincreasing/decreasing as expected due to discontinuities in the emitterantenna model. According to one embodiment, these discontinuities areidentified and errors are flagged if the correction would be largeenough to imply an error in the model. These errors may be created, forexample, during emitter input (e.g., a data entry error) may be due to adata integrity error, or other reason. In conventional detectionsystems, such errors are not detected and if unrecognized, would lead toan inefficient or erroneous dwell strategy.

More specifically, it is assumed that the antenna structure of anemitter will have a main beam which is several degrees (or fractions ofa degree) wide, and a sidelobe structure situated on either side of themain beam. These side lobes will have lower magnitude moving away fromthe main beam, and this magnitude drops sharply as the distance from themain beam becomes greater.

If a system used to detect such an emitter sees an atypical lobingstructure, the emitter may be ignored as the antenna model is incorrect(and therefore the emitter should not be used to determine the dwellsolution). For instance, using the antenna model described above, it isexpected that as the sensitivity of the receiver is adjusted to detectmore of the model (e.g., sidelobes), the revisit time for detectingshould also increase. If the revisit time decreases as sensitivityincreases, an error may exist in the antenna model.

FIG. 5 shows a process 500 for evaluating an antenna model according toone embodiment of the invention. This process may be performed, forexample, by a detection system (e.g., system 201). At block 501, process500 begins. As part of determining dwell duration, dwell revisit time(RVT) is computed for each detecting method (e.g., IF and videobandwidth combinations) of the receiver. At block 502, a sub-matrix maybe created for each emitter/emitter mode processed, each rowrepresenting the results for each emitter/mode, and each columnrepresenting one of the detecting methods. At block 503, the columns areordered by increasing sensitivity of the detecting method. Because thecolumns are in increasing sensitivity order, physics dictates that thecomputed revisit time (RVT) across the columns should be equal ormonotonically increasing. Therefore, according to one embodiment of theinvention, a method is provided that determines whether revisit timedecreases as sensitivity increases, and if so, an error is identified.

According to one embodiment, an algorithm is provided that analyzes theemitter matrix for errors in an antenna model. In one example shown inFIG. 5, the detection system loops through each row of the sub-matrixand compares the N^(th) non-zero value of RVT to N+1 value, to identifya decreasing delta (in one embodiment, a value of zero is used toindicate that no RVT computation was performed for the correspondingdetection method). In particular, the detection system evaluates foreach N^(th) non-zero value of RVT, the difference between the values ofRVT_(N) and RVT_(N+1) at block 504. At block 505 it is determinedwhether the value of RVT is increasing, and if so, the N+1 column isdetermined to be valid, and N is incremented at block 507. If the valueof RVT is decreasing the N+1 column is marked as invalid, and the N^(th)column is compared with the N^(th)+₂ column to determine whether RVT isincreasing. If no discrepancies are found, then the column of thesub-matrix is valid.

Optionally, if the deltas are small enough, the column values are put inascending RVT order, under the assumption that the error is due to aminor modeling discontinuity. An error may be considered “large enough,”for example, depending on how good the antenna model is (e.g., whetherthe antenna model is accurate in its description of the lobingstructure) and how closely the detection system should adhere to thismodel. For example, if the antenna model is determined empirically fromdata, and data points are interpolated, a less-stringent error may beneeded to account for minor errors in the model. However, if the modelis based on range testing or some other more accurate method, then theallowable error may be less. According to one embodiment, this error isconfigurable by an operator according to the antenna model used. Forinstance, the model could be considered valid if error is not greaterthan 30%. However, it should be appreciated that the error value couldbe adjusted to any acceptable value to identify model errors withouttriggering false indications, and that the invention is not limited toany particular value.

If the detected error is large, no correction is made and the error isleft for downstream validation to detect and flag to the operator (e.g.,via interface 203). In one embodiment of the invention, the algorithmmay be performed as part of process 300 for determining a scan strategyas discussed above. In particular, the algorithm may be performed aspart of, for example, block 302 wherein emitter data is input, and ischecked for discontinuities prior to determining a dwell strategy.

FIG. 6 shows another process 600 for evaluating an antenna modelaccording to one embodiment of the invention. At block 601, process 600begins. According to this embodiment, the evaluation is segmented bycolumns at block 602, allowing comparisons across groups of relatedcolumns, and therefore the detection system can perform comparisonsbetween groups of related detecting methods. For each group, thedetection system evaluates the entries within the group at block 603.This evaluation for a subgroup may be similar to the evaluationperformed on an entire group as discussed above with reference to FIG.5. In one embodiment of the invention, seven columns are checked in twopasses, one pass testing the monotonic relationship across the firstfour columns representing a first group, and the other pass testingacross the last three columns representing a second group. According toone embodiment, detecting methods within a group may be reordered withinthat group. Discontinuities among the groups can also be identified(e.g., at block 604). In one embodiment of the invention, if there is adiscontinuity between groups, it is left uncorrected to be flagged bydownstream validation logic for the operator's resolution or resolutionby another process. For instance, at block 606, the emitter model may beflagged as being suspect for resolution by the operator or anotherprocess. At block 607, process 600 ends.

According to one embodiment of the invention, evaluation of error may beperformed within video bandwidth (VBW) groups. More particularly, groupsmay be placed, for example, in ascending order, and errors determined bycomparing consecutive entries. As discussed above, entries having adecreasing value within the ascending order may be ignored, while errorsdue to minor sensitivity deltas may be corrected. Errors between IFbandwidths may also be detected, however, the error may be coarser, andtherefore the error tolerance greater between entries.

In this manner, the accuracy of the emitter database is increased, andas a result, the scan strategy based on the more accurate database ismore accurate. More particularly, errors in the emitter database areeliminated and removed prior to determining the scan strategy, andtherefore the scan strategy is more accurate.

Multi-valued Illumination Time Revisit Time Calculation

As discussed above, an emitter may, for example, present more than oneillumination time to a detecting receiver. For example, a multifunctionradar that both sweeps azimuth and changes its elevation angle presentsmultiple illuminations to be detected. In this example, each pass of theradar provides a different power level beam width to the receiver thatshould be detected.

According to one aspect of the present invention, a method is providedfor computing the revisit time for an emitter that presents more thanone unique value of illumination time to the detecting receiver. In oneembodiment, each of the multivalued illuminations are represented as asingle emitter, and a revisit time is calculated that meets requirementsfor detecting any of the illuminations.

The revisit time equation is a closed-form equation when illuminationtime is single valued, but needs to be solved iteratively when it isnot. An inefficient approximation is to take the average of theillumination time values and use the closed form equation. Conventionalsystems generally use the average value and it is realized that thisaverage value is not, in general, the most efficient solution. Accordingto one aspect of the invention, a detection system uses a more weightedmethod for determining revisit time for multivalued illuminations, inone embodiment, an open-form equation is used to determine acorresponding revisit time for multivalued illuminations. Therefore, oneaspect of the invention involves solving the open-form equation in aniterative manner within a tolerance (e.g., an acceptable probability ofdetecting each illumination). In one embodiment, an initial RVT isdetermined based on an average value of the time in beam (TIB) of all ofthe illuminations of interest. The average value of RVT may be, in mostinstances, a good “first guess” at an RVT value which can be improvedupon iteratively. For instance, in one embodiment, the initial RVT maybe estimated (e.g., by taking the average value) and then adjusted tomeet a desired probability of detection.

In one embodiment of the invention, dwell revisit time (RVT) isdetermined for each detecting method (e.g., IF and video bandwidthcombinations) of the receiver. This computation involves evaluating thefollowing equation:

$\begin{matrix}{{RVT} = {{TIB}\left( {1 - \frac{N}{{Ln}\left( {1 - {Pd}} \right)}} \right)}} & {{Equation}\mspace{20mu} 1}\end{matrix}$

Where N is the number of detection opportunities (e.g., allowable radar“paints”), Pd is the desired probability of detection and TIB is theexpected duration of each “paint” or illumination. TIB is only a singlevalue for 2D scans under static conditions. For 3D scans, there may beseveral values of TIB per “paint” (N), and under dynamic scenarios, 2DTIB increases/decreases on subsequent paints based ondecreasing/increasing range, respectively.

The following example process determines RVT with several TIB values perpaint N and varying TIB values per paint N. This process includes anassessment of the partial contribution of each TIB in satisfying theoverall probability of detect, and incrementing/decrementing the trialRVT value until the sums satisfy the desired probability of detection,within a tolerance.

Given:

-   -   TIBS: A vector of illumination times for the observation period    -   Pd: Desired probability of detection    -   STEP: Minimum RVT increment    -   RVT_(max): Maximum permissible RVT value

The following process may be used to determine RVT:

-   1. Initialize the constants NI and PPT.    NI:=integer number of “paints” in the observation period    PPT:=number of elements in TIBS

These represent the total number of passes the radar makes across thesearch volume, and the total number of paints within that are observablewithin the search volume across the observation period, respectively. Ifthe TIBS data represents 3D scan information, these constants areadjusted as follows:PPT:=PPT÷NI (paints per volume search)NI:=I

-   2. Using Equation 1 above, compute the initial RVT by setting N to    NI and TIB to the average value of TIBS. If the resulting RVT is    larger than the largest value in TIBS (and therefore the average    value of TIBS may not be the best “first guess”), recompute RVT with    TIB set to the max value of TIBS and N=1.-   3. RVT is now that initial solution. Determine the search direction:

${SIGN} = {{\ln\left( {1 - {Pd}} \right)} - {\sum\limits_{M = 1}^{PPT}\frac{NI}{1 - \frac{RVT}{{TIBS}_{M}}}}}$Set SIGN to +1 or −1, depending on if it is positive or negative.

-   4. Determine if RVT value is close enough, exiting if the predicted    Pd delta is within tolerance:

${Tolerance} \geq {{Pd} - {\mathbb{e}}^{\sum\limits_{M = 1}^{PPT}\frac{NI}{1 - \frac{RVT}{{TIBS}_{M}}}}}$

-   5. Increment/decrement trial RVT:    RVT=RVT+SIGN×STEP    Exit if RVT increments/decrements out of the range 0≦RVT≦RVT_(max),    limiting RVT to the crossed bound.    If the loop counter exceeds RVT_(max)÷STEP, exit logging an error.-   6. Test trial RVT:

${SIGN}_{T} = {{\ln\left( {1 - {Pd}} \right)} - {\sum\limits_{M = 1}^{PPT}\frac{NI}{1 - \frac{RVT}{{TIBS}_{M}}}}}$Set SIGN_(T) to +1 or −1, depending on if it is positive or negative

-   7. If SIGN equals SIGN_(T), repeat steps 5 and 6. Exit if SIGN_(T)    is positive. If SIGN_(T) is negative, decrement RVT by STEP and    exit.

By using the above process, an RVT is determined that takes into accountmore than one unique value of illumination time, and is more efficientthan computing an average value using all of the illuminations ofinterest. Because the computed RVT satisfies a predetermined tolerance,the probability of detection for each of the illuminations is satisfied.

Multiple Intercept Rule Evaluation

As discussed above, there are conventional detection systems that scan arange of frequencies linearly and therefore these systems do notestablish scan strategies. However, in a system that computes a scanstrategy having multiple solutions for establishing dwells, multiplescan strategies may be determined having multiple configurationpossibilities for one or more receiver(s). That is, there may be morethan one scan solution for a given set of emitters, and there may bemore than one hardware configuration that meets each scan solution'srequirements. According to one aspect of the invention, a method isprovided for solving the combinational problem for configuring receiverhardware where more than one possible scan solution exists.

Problems also exist in determining a solution for detecting multipleemitters involving satisfying the scanning requirements for each emitterand determining an overall solution. In particular, the detection systemestablishes, for each emitter, at what time and what range the signalproduced by the emitter should be intercepted. Also, the system mustdetermine how fast to sample the incoming signal to achieve anacceptable intercept time. Because the solution for multiple emitters isa complex problem, a system that has flexibility in prioritizing andhandling the detection of multiple emitters is preferable.

According to one aspect of the invention, a rule-based system isprovided for determining how emitters should be detected by a detectionsystem (e.g., detection system 201). According to one embodiment of theinvention, rules are associated with particular emitters which determinehow signals generated by corresponding emitters are detected by thedetection system. For instance, rules may be used to prioritize certainemitters with respect to other emitters and, based on these rules, anoperator of the system may determine a solution to the scanning problem.

In one embodiment, there may be parameters that may be associated withan emitter of interest that affects if and/or how the emitter isdetected. These parameters define generally how each rule operates. Inone embodiment of the invention, an intercept rule may be defined peremitter mode, the rule determining, for example, at what minimum rangethe emitter needs to be detected, the allowed probability of interceptof the emitter, the amount of time allowed to observe an emitter priorto detection, etc. According to another embodiment, this may be expandedto allow several rules per emitter mode, and to allow for automaticselection of the most appropriate rule given a resolution criterion(e.g., most stringent rule, least stringent rule, ignore particularrules, etc.).

As discussed above, FIG. 3 shows a flow chart for determining a scansolution for emitters of interest. The first step of this processdatabase determine an emitter of the emitters of interest for which asolution is determined. For each emitter and emitter mode, a single setof detection rules may be defined (e.g., by a user or operator). One ormore detection rules may be stored in the emitter database (e.g., in aninformation matrix) and one or more rules may be associated with anemitter entry.

According to one embodiment of the invention, a detection rule may becreated that includes one or more of the following parameters:

-   -   Probability of Intercept (Pd). This defines the probability or        confidence that the signal will be detected within the desired        time period, expressed in a number of scan periods (“paints”) or        clock time.    -   Turn-on Range. Maximum range from the receiver at which the        signal becomes interesting to the receiver.    -   Detect-by Range. Minimum range by which detection is required to        have occurred at least once.    -   Tolerance. Enumerated type to select observation time as        time-of-flight between turn-on and detect-by ranges; number of        scan periods; time; or the larger of time/scan periods.    -   Tolerance Direction. Indicates whether a tolerance is applied        prior to or following the detect-by range.    -   Scan Periods. Indicates the number of emitter “paints” or        illuminations that are allowed for observation prior to        detection.    -   Time. Indicates the amount of time allowed for observation prior        to detection.    -   Cumulative Pd flag. Enables Cumulative Pd logic. This logic        basically accounts for any signal amplitude change as range        changes from turn-on to detect-by ±tolerance. Amplitude may be        considered constant as computed at turn-on range when the flag        is false.

These parameters may be used to establish the geometry between thereceiver and the emitter, and ultimately compute the required revisittime for each detection method of the receiver. Multiple sets ofdetection rules may be used, because an operator may have differentscenarios in mind. For example, six sets of rules may be supported. Thelimit of the number of allowed rule sets may be set by human factors.The operator does not have to populate all six sets, but may choose topopulate sets one through six, in any order. To support the multiplerules, two additional parameters may be added to the database peremitter, emitter mode definition:

-   -   Chosen Rule. This parameter allows the operator to pick a        particular detection rule set, and allows a detection system        (e.g., system 201) to resolve a solution across the detection        rules. Therefore, an operator has the flexibility to choose        certain rules to be used for a particular emitter.    -   Cross Rule Relation. This parameter allows the operator to guide        the detection system to the appropriate rule:        -   Do not scan for the emitter mode at all (overrides all            detection rules).        -   Assign a minimum cost, default revisit time (overrides all            detection rules).        -   Evaluate all populated rules, and select the most difficult            across the detection methods.        -   Evaluate all populated rules, and select the least difficult            across the detection methods.

Given a set of data (the set referred to hereinafter as “DATA”)specified by a matrix of emitter parameters (e.g., the informationmatrix) including multiple intercept rules, one example method forevaluating the detection rule sets may be performed as follows:

-   1. Loop on each row (emitter) in set DATA.-   2. If the value of the Cross Rule Relation parameter indicates “Do    Not Scan”, skip processing of this emitter.-   3. If the value of the Cross Rule Relation parameter indicates    “Default”, then assign default parameters.-   4. If the value of the Chosen Rule parameter indicates a specific    detection rule set, process the selected rule set for the emitter.-   5. Otherwise, there are multiple detection rule sets to evaluate the    row:    -   a. For each populated detection rule set, replicate the emitter        row and process the rule set for the new row.    -   b. For each replicated row, assign a common identifier (e.g., a        tracking index) to identify the replicated data set.    -   c. Compute the information matrix for DATA, including the        replicated rows. This will result in the computation of Revisit        Time (RVT) for each receiver detection method for each row        (original and replicated).    -   d. Using the common identifier (e.g., the tracking index),        identify the unique sets of RVT data that resulted from the        multiple rule sets. This data can be visualized as a matrix of        RVT values, where each column represents a detecting method, and        each row represents the emitter evaluated for each rule. The        data may be consolidated into a single set of RVTs based on the        Cross Rule Relation selection (Most or Least Difficult) using        the following procedure:        -   i. If “Least Difficult” selected, then            -   1. Ignore rows with all zero values (i.e., no detection                using such rules possible).            -   2. If there are rows with all non-zero values, select                these. Otherwise use all remaining rows.            -   3. Loop through the columns in increasing sensitivity                order.                -   a. Identify the row with a unique maximum RVT value.                    If found, this is the row with the easiest solution.                -   b. If all columns are checked and the no unique                    maximum is found, select the first row found on the                    last “pass” as the least difficult solution.        -   ii. If “Most Difficult” selected, then            -   1. Ignore rows with all zero values (i.e., no detection                using such rules possible).            -   2. If there are rows with all non-zero values, select                these. Otherwise use all remaining rows.            -   3. Loop through the columns in increasing sensitivity                order.                -   a. Identify the row with a unique non-zero minimum                    RVT value. If found, this is the row with the most                    difficult solution.                -   b. If all columns are checked and the no unique                    minimum is found, select the first row found on the                    last “pass” as the most difficult solution.        -   iii. Insert the selected RVT data into the information            matrix, replacing the data of one of the elements of the            common tracking index set, and deleting the remaining            replicated data of the tracking index set.

In this manner, rules may be used by an operator to control how thesystem prioritizes and monitors emitters of interest. That is, theoperator is provided the capability of determining which emitters and inwhat priority these emitters are detected. These rules may also used bya detection system to automatically prioritize emitters when determininga scan solution.

Multiple Hardware Configuration Evaluation

As discussed above, a system according to one embodiment of theinvention may be capable of determining more than one scan solution tosatisfy a given set of emitters. Similarly, there may be multiplehardware settings that satisfy a given dwell solution.

In conventional systems, there is typically one hardware configurationappropriate for signal sampling. According to one embodiment of theinvention, a system may be provided (e.g., system 215) that allows anoperator to set alternate sampling configurations which provideequivalent representations of the intercepted signal. Thus, in oneembodiment of the invention, representations of multiple hardware (HW)configurations per emitter mode may be used, and the most appropriatehardware configuration for the intercepting dwell may be selected. Inone embodiment, configurations may be created and stored in the emitterdatabase (e.g., information matrix) where they can be used later indetermining an optimum dwell set.

According to one embodiment, the system may define one set of hardwaresettings for a particular receiver. This capability may be expanded, forexample, to multiple sets of hardware settings. This additionalcapability may be useful, for example, for allowing the operator todefine and make a final selection from the multiple sets of hardwaresettings that satisfy the dwell solution. For instance, differentemitters may demand conflicting hardware settings, and allowing anoperator to choose among multiple configurations can avoid such hardwareconflicts. Conflicts may, for example, be flagged by the detectionsystem as errors. However, because the detection system is capable ofdetermining multiple set of hardware settings to choose from, thepossibility that a conflict exists is less likely to occur.

Optionally, the detection system may be configured to present and/orselect from multiple hardware configurations that do not have conflicts.For example, a cost may be associated with each hardware configuration,and the most appropriate configuration may be selected based on itscomparative cost with other hardware configurations. Other ways ofselecting optimum hardware configurations may be used, and the inventionis not limited to any particular selection method. For example, theconfiguration that requires the smallest dwell duration may be preferredover other configuratives.

A hardware configuration may include various hardware controls that areconfigured to operate the receiver hardware. These controls may include,for example, a set of controls referred hereinafter to as discretecontrols. Examples of discrete controls may include:

-   -   POP Control. A boolean value that enables/disables hardware        receiver function.    -   Chop Control. An enumerated value that allows the operator to        choose among discrete values for “chopping” the incoming signal        as well as disabling the chop control. This control allows the        operator to chop the incoming signal into pulses.

There may be, for example, sets of other controls that correspond tofiltering operations that affect how a pulse train of the receivedemitter signal may be perceived by upstream receiver components (e.g.,software used to analyze the received pulse train). The operator maywish to control how the received signal is processed by these upstreamcomponents. A set of controls referred to hereinafter as range controlsmay be provided and this control capability may include, for example,one or more of the following controls:

-   -   Doppler Trigger Count (min/max values). Upper and lower bounds        of acceptable pulse counts which may trigger doppler processing.    -   Pulse Grouping Interval (min/max values). Upper and lower bounds        of acceptable pulse interval timing that allow correct pulse        repetition interval (PRI) measurements.

As discussed above in creation and evaluation of multiple interceptrules, the emitter database 210 is expanded to allow multiple sets ofrules per emitter data element, and the operator may populate one ormore of the sets with rules. Likewise, multiple hardware configurationsettings (e.g., may be defined and stored by the detection system inemitter database 210 used to determine the optimum hardware solution.

Determining Conflicts

To understand how conflicts occur, it is necessary to understand how theemitter signals are observed and processed. For example, in the N×Minformation matrix discussed above, MDT for an emitter may be definedminimally as one Pulse Repetition Interval (PRI), and EDT may be definedas N*PRI of the signal represented in a row. This signifies that thedetection system executes the dwell every RVT, and “sits” there for aperiod of the MDT to observe one pulse. If nothing is detected, thesystem moves onto the next dwell in the sequence and “sits” for a periodof MDT. However, if pulse activity is detected, then the dwell extendsobservation of the signal up to EDT to collect the desired number ofpulses (N). N may be chosen based on signal processing considerations,and may range, for example, between 3 and 20 pulses, although otherminimum and maximum values may be used. Considerations for determiningthe minimum number of pulses include averaging measurements made on eachpulse; the maximum number of pulses may define the volume of datarequired to analyze the signal, as necessary.

As discussed above, the MDT may be minimally one PRI, but there areexceptions that may alter this relation. Therefore, there are multiplepairs of MDT/EDT, and these pairs of MDT/EDT are driven by multiplepairs of “Pulse Sort Sets” PD_Trigger/Intra_Group ranges, respectively.These ranges provide a set of valid signal conditioning options that maybe selected for the dwell to process the signal correctly. Pulses may betransmitted in groups, and the receiver would like to define PRI as thetime from “first pulse in group” to next “first pulse in group” to makesignal processing easier.

The Intra_Group parameter shown in FIG. 7 defines the “PRI” range overwhich pulses may be grouped; the PD_Trigger parameter defines theexpected number of pulses in a group. Particular values selected forthese ranges may alter the corresponding values of MDT/EDT. Problemsarise when “real” PRIs of one emitter fall into the grouping range ofanother. In that case, the trigger count may be exceeded. The receivermay be configured to recognize this and may modify the grouping logic,but depending on implementation, may need to collect an additionalPD_Triggers worth of pulses. In this case, MDT actually may representthe time it takes for the detection system to collect an additionalbatch of pulses.

As shown in FIG. 8, “Pulse Sort Sets” are used to condition the pulsetrain for signal processing, particularly with respect to pulse-groupedsignals. The consequence of this, however, is that it is possible forpulse sort values to satisfy one emitter and conflict with another.Hence, multiple hardware configurations may be supported. The settingsthat satisfy all or most of the signals may be selected (e.g., by thedetection system or alternatively by the operator). The detection systemmay also display an error if not emitters all can be satisfied. MultiplePulse Sort Sets lead to corresponding MDT/EDT sets, the effect of thePulse Sort Sets may cause more pulses to be collected per dwell.

The following are several examples illustrating how conflicts can ariseand be detected by a detection system:

EXAMPLES

The POP Boolean control is either on or off, so conflicts are handled bysorting dwells around the conflict, or if that is not possible, flaggingthe conflict. This is illustrated by the following example:

Emitter #1 Frequency range: 1500-2000 MHz, POP On (normal case). Emitter#2 Frequency range: 2050-2550 MHz. POP Off (for some receivers, this isrequired for AM pulses, since the assumption of “square” pulses is nottrue, and may cause multiple encoding if POP is left enabled).

Assume a detecting bandwidth of 1000 MHz and 250 MHz. Any solution thatattempts to include the emitters in the same dwell will cause a conflictto be detected. Such solutions are not favored by the dwell placementmethods described below, which should find at least two 1000 MHz dwellsat bandwidths 1000-2000 and 2050-3050. These two dwells avoid theconflict. If there is no gap between the dwells, or if additionalconflict exists, then the 1000 MHz dwell may not be allowed.

For instance, expanding on the previous example, add an Emitter #3 withPop On and a frequency range 1000-1450 MHz. There is no 1000 MHzemitter, so therefore their may be a conflict-free solution to coveremitter #2, so one viable solution would include creating a dwell atbandwidth 450-1450, another at 2050-3050 and a pair of 250 MHz dwells tocover emitter #2, at bandwidths 1500-1750, 1750-2000.

Therefore, for the Boolean conflict discussed above, the process willend up finding any “gaps” between the conflicts and attempts to builddwells above and below the gaps to separate the emitters. This cannot bedone in every case, and certainly cannot be done if the conflictingemitters overlap in frequency.

If the conflict cannot be avoided, the conflict may be recorded for theoperator, since it might be possible to alter the emitterrepresentation. If the operator cannot alter the representation, thenthe dwell solution will be redundant in the overlap range, with at leastone dwell each tailored to solve the conflict. Building on the previousexample, assume emitter #2 was identical to emitter #1 1500-2000, butPOP is still off, and there is no emitter #3. Then two 1500-2500 MHzdwells are created, one with POP on and one with POP off, and an erroris logged. A 3^(rd) dwell is still needed to cover emitter #1.

Chop is an enumerated type of short, long and “don't care”. If allemitters are “don't care” then the default value, usually short, isassigned. The expectation is that most emitters are “don't care”, soselection of a value other than the default is driven by the “rare”emitter that wants “long” or “short”. If there is a conflict between“short” and “long”, then the problem is very much like the Boolean POPcase, and is solved the same way: use bandwidths that best isolate theconflicts, and in the absence of being able to do that, create redundantdwells for the conflicting region and log the problem.

For Doppler_Trigger and Pulse Grouping, the situation has more options.Each of these has an acceptable min/max range from which a value can bepicked. In addition, additional sets of values can be assigned in thedatabase by the operator (e.g., a total of six sets). The system selectsa value for each of these fields that satisfies one of the six sets,favoring the minimum values. Referring below to the following example inTable 1:

TABLE 1 Set #1 Set #2 Pulse Pulse Pulse Pulse Doppler Doppler GroupGroup Doppler Doppler Group Group Emitter Min Max Min Max Min Max MinMax Emitter #1 2 12 20 μsec  50 μsec Emitter #2 3 15 40 μsec 100 μsec

In this example, only one set per emitter is defined. The assignedvalues for the well is 3 and 40 μsec, since these are the minimum valuesthat satisfy both emitters. Now refer to the following example below inTable 2:

TABLE 2 Set #1 Set #2 Pulse Pulse Pulse Pulse Doppler Doppler GroupGroup Doppler Doppler Group Group Emitter Min Max Min Max Min Max MinMax Emitter #1 2 12 20 μsec  50 μsec 16 20 20 μsec 50 μsec Emitter #2 1420 40 μsec 100 μsec 22 25 60 μsec 75 μsec

In the example in Table 2, the numbers are assigned such that there isnot overlap within a set. In this case, the values 16 and 40 μsecsatisfy emitter #1, set #2 and Emitter #2, Set #1. These dwellparameters imply that pulse trains with repetition intervals of lessthan 40 μsec are considered pulse grouped, and it may take 16 pulses torecognize a Doppler signal. This is acceptable relative to the database,because this is entered as a valid option. If emitter #1 was by itself,then these numbers could have been reduced to 20 μsec & 2. If there ismore than one viable solution, the one that requires the fewest pulsesand therefore requires the smallest dwell duration is selected.

If however, no overlap can be found among the hardware configurations,then the solution may be pushed by the improved dwell placement methodsdescribed above to find any frequency “gaps” to exploit. Barring thissituation, redundant, overlapped dwells are generated to providecoverage and the conflict is logged for the overlapped region/dwells.One difference here is that the selected values are the ones thatsatisfy the most emitters. For example, if there are 10 emitters and asingle set of values for a dwell can satisfy 9 of the 10 emitters, thatset of dwell parameters is selected, and a separate dwell is built forthe 10^(th), “oddball” emitter.

Hardware Control Evaluation

When a dwell is hypothesized for a given frequency range, the emitterswithin the range to be processed by the dwell may be examined toestablish the HW parameters for the dwell and detect and resolveconflicts. This may be done, for example, in parameter “flexibility”order, beginning with the most flexible parameters, which are the rangecontrols discussed above.

The dwell is assigned a single value of Trigger Count and a single valueof Grouping Interval. The selected value lies within the range of atleast set of the emitter to satisfy the emitter. In addition, theTrigger Count and Group Interval are “coupled”, meaning that bothparameters of the set bracket the selected values to “count”. A conflictexists if the selected values do not satisfy any set of at least oneemitter. The selected values are the minimum values that satisfy allemitters, or the preponderance of satisfied emitters. An example isillustrated in FIG. 9, where “X” indicates selected dwell values. In theexample shown, three sets of range control parameters are shown, anddwells are selected that satisfy all emitters.

Next the discrete controls may be set using the emitters that aresatisfied by the range control selection. Emitters that are indicated as“don't care” for a given parameter do not contribute to that parameter'sselection. Again, the parameter is set to the value that satisfies thepreponderance of the emitters. If all emitters indicate “don't care” fora parameter, then the value is set to a predetermined default value.

An indication of the number of controls experiencing conflicts is kept.If the dwell is kept as part of the scan strategy, the number ofunresolved conflicts is taken into account in selection of the mostappropriate scan strategy (e.g., the one with the fewest conflicts, thenthe lowest cost).

The selection process begins by trying to resolve Doppler Count/PulseGroup set first for the preponderance (if not all) of the emitters inthe dwell. Mechanically, this may be accomplished by the followingexample:

-   1. Placing each of the sets into a single seven-column matrix, whose    seven columns are Doppler min/max and Pulse Count min/max, MDT, EDT    and emitter ID, respectively. The Emitter ID field allows the    tracking of the multiple to the emitter “owner” of the set. A vector    of unique ID values is saved in the matrix referred to as “ID”.-   2. Any rows that have unpopulated sets are discarded from the    matrix. These are the ones that have zero values in the    Doppler/Pulse Group columns. The matrix is now an N row by 7 column    matrix.-   3. Evaluate each Doppler Min (column 1) for containment in each    range Doppler min≦Doppler Min≦Doppler Max, creating a N×N matrix of    Boolean solutions. Because Doppler Min will always “pass” its own    range, the matrix diagonal contains a value of 1. (Note that if all    the other matrix values are zero, PD ranges have no intersection).    This may be referred to as the “I” matrix.-   4. Create a N×N matrix, which replicates the Emitter ID field across    each row (column 7 of the ID matrix created in step 1). Multiply    this matrix by the “I” matrix. The effectively replaces the “1”s in    matrix I with the corresponding ID numbers. Reassign this resulting    matrix to the “I” matrix.-   5. Loop on each element in the ID vector and test for ID[k]=I. This    creates k N×N Boolean arrays. Visualize this as a 3-D array, with    N×N being the x-y plane and k is the z-axis.-   6. OR the 3-D array across the x-axis. This results in a Boolean    array of dimensions N×k. Perform a sum across the columns, creating    an N element vector. Each element is the count of emitters the Nth    Doppler min/max range solves for.-   7. Identify the position(s) in the N element vector that has the    largest count. This identifies the row(s) of the multiple column    matrix of step 2 that solve Doppler count best. If the “largest    count” is not unique in the vector, then select the Doppler count    value that is mapped to the smallest MDT value (i.e., basically,    sort on column 5).-   8. Now solve for Pulse Group Interval:    -   a. Given the Doppler Count solution of step 7, reduce the        7-column matrix of step 2 to the rows that satisfy Doppler        min≦Doppler Count≦Doppler Max. This may return all or a subset        of the matrix.    -   b. Repeat steps 3-7, but extract Pulse Group Interval rather        than Doppler Count.-   9. Now assign MDT & EDT:    -   a. Create another “I” matrix as in step 3 above, which satisfies        the selected Doppler Count and Pulse Group Interval by        evaluating [Doppler min≦Doppler Count≦Doppler Max] AND [Pulse        Group min≦Pulse Group≦Pulse Group Max]. Log a conflict if any ID        is not satisfied by this selection. This may be performed, for        example, by incrementing a global counter that tracks conflicts        (e.g., a “Doppler/Pulse Count conflict counter”).    -   b. Perform steps 4-6 on this “I” matrix.    -   c. Perform step 7 above, but inspect the “I” matrix for an        MDT/EDT entry that corresponds to the “largest count”. If the        “largest count” is not unique, then MDT and EDT may be set to        the largest values in the solution set.

Step 8 determines the emitters to be solved for, step 9 establishes, MDTand EDT. The emitter list is then tested for compatible POP and Chopvalues. The POP and Chop values are set to the values that satisfy themost emitters in the set, and any conflicting emitters are dropped fromthe set. Dwells for these emitters will be build on a subsequent pass.Again, global conflict counters are maintained for POP and Chop.

As discussed above, cost may be used to determine the most appropriatescan strategy. A cost may include, for example, overall evaluation ofthe dwell solution. At the start of the process, the global conflictcounters are zeroed out. As each dwell is built, dwell parameters, andincrements the counters as conflicts are encountered. At the completionof each candidate scan table, the cost of dwell execution (Sum of Dwelldurations/Revisit times;) and total number of conflicts is compared tothe results of the prior scan table build pass, and the one with thelesser conflicts is kept as the solution. If the number of conflicts isequal, then the lesser-cost solution is kept. The latter may be thetypical case, if solutions exist around potential conflicts.

Real Antenna Data Option

A detection system that detects one or more emitters may use antennadata that describes the characteristics of various antennas used by suchemitters. This data is then used by a detection system to identify andclassify emitters encountered in the field. Conventionally, antennameasurements are performed which yield such data in a facility referredto in the art as an antenna range. An antenna range is generally anelaborate and a costly facility (e.g., an antenna range, anechoicchamber, etc.) that isolates an antenna from external energies (e.g.,range clutter) and allows for the measurement of antenna characteristicssuch as beam width, gain, sidelobe levels, and polarization of antennasor antenna subsystems over a particular frequency range. A detectionsystem that uses such data is limited by the number and type of antennasdefined to the detection system, and is limited in identifying antennasthat are unknown or are undefined to the detection system. Further, itis realized that static antenna characteristics measured usingconventional methods do not adequately define all antennas, even of thesame type. For instance, two antennas of the same type may havedifferent antenna characteristics which may cause them to be seen by adetection system as different antenna types.

According to one aspect of the invention, it is realized that it wouldbe beneficial to use antenna characteristics measured by the detectionsystem to model antennas. Because antenna models can be “learned” andused by the detection system to detect those antennas, the detectionsystem capabilities are increased.

Also, because actual data from antennas measured in the field can beused by the detection system, the detection system may be more accurateas a result. It is realized that conventional databases of antennamodels contain coarse data that describes a particular model, andtherefore the detection system is limited to using this coarse data todetect emitters. More particularly, in conventional detection systems,models are empirically defined using data supplied by conventionalsources (e.g., the RAND Corporation) and a portion of the data thatdescribes these models is estimated. However, in a system that canobserve antenna characteristics in the field, the detection system isnot limited to coarse data provided by a static antenna model; thedetection system is capable of determining more accurate models formodeling antennas. For example, measured data may yield models that moreaccurately determine the curvature of an antenna lobe pattern.

A set of models may be used to compute antenna characteristics as afunction of frequency, gain, power, beam width, scan and polarization.Thus, actual antenna gain versus azimuth may be observed by thedetection system (e.g., detection system 201) for several polarizations,and this data may be reduced for antenna modeling purposes.

The emitter database may include a field that allows the specificationof a location where “real” antenna data can be found for the emitter. Ifa location is specified, the “real” antenna data may be accessed andused instead of the internal antenna model. The antenna data may be onefile for several emitter modes, or a unique antenna file per mode. Ifdata cannot be found or is invalid, an error may be logged by thedetection system and the internal antenna model is used. If data isfound, then illumination times may be determined directly from the realantenna data.

For example, given a set of data specified by a matrix of emitterparameters (referred to hereinafter in the example below as “DATA”),each row representing an emitter, emitter/mode and set of real data(referred to hereinafter in the example below as “RealData”), specifiedby actual antenna data (e.g., amplitude (dBm) vs time for one or morepolarizations, representing at least one scan period), an example methodfor determining illumination times may be performed as follows (e.g.,when computing RVT for the Nth emitter in DATA):

-   1. If the emitter indicates that RealData is available, access the    real antenna data file.-   2. If the file does not exist or has invalid data, log an error and    resume with the internal antenna model.-   3. Otherwise, proceed to access and process RealData.    -   a. The file may contain up to four polarization curves:        Horizontal, Vertical, LHC and RHC. Select Horizontal (H) &        Vertical (V) if both valid, or Left Hand Circular (LHC) & Right        Hand Circular (RHC) if both valid. Otherwise, select the first        valid polarization found.    -   b. Adjust amplitudes of each valid polarization for frequency        dispersion due to the delta between the frequency of the data        and desired frequency of the emitter.    -   c. Apply receive antenna polarization loss model to RealData.    -   d. If there is more than one polarization, merge the        polarization data as the larger value for each time point.    -   e. Interpolate any missing amplitude points versus time.-   4. RealData now includes time versus amplitude data. Adjust    amplitude for the range dispersion loss.-   5. For each detection method to be evaluated:    -   a. Screen out amplitude points below the method's sensitivity.    -   b. Cross-correlate the extended dwell time (EDT) with the data        to determine the unique illumination times (TIB) of the data.        This basically “slides” an EDT rectangle cross the data in time,        recording time above the method's sensitivity level. Each        illumination time represents a unique intercept opportunity of        sufficient duration to constitute a potential detection. An        example shown in FIGS. 10A-10B shows TIB extraction for two        different sensitivity levels that yield two different sets of        discrete illuminations (TIBs) to be detected. Time is measured        at or above a particular sensitivity level, as data below the        sensitivity level are ignored.    -   c. Scale TIBs to the ratio of recorded scan period to the        desired emitter scan period.    -   d. Compute RVT for the detection method (e.g., via the        multi-valued illumination time RVT calculation discussed above).

It should be appreciated that other methods may be used to measure andprocess real data from one or more emitters, and the invention is notlimited to any particular method. In this way, the system may use moreaccurate information that can be measured from real emitters, if thatdata is available. If real data is not available the detection systemmay use empirical data provided by one or more sources.

Jammer Compatibility

In one embodiment of the invention, a detection system (e.g., system201) may operate in a manner cognizant of on-board active jammers (e.g.,Electronic Countermeasures (ECM) systems) for jamming or misleadingenemy weapons, communications, or radar. More specifically, thedetection system may take into account the operation of one or morejammers when determining a scan strategy and may optimize jammer bandand intercept band performance as a result. Conventionally, jammers anddetection system hardware operate independently, and therefore, when anactive jammer is operated, a detection system configured to detectemitters within or in proximity of the frequency band in which theactive jammer operates produces a false indication of a threat. Further,because operation of the jammer does not take into account the interceptrequirements of the detection system and therefore the detection systemcannot perform detecting functions in the same frequency bands that thejammer is operating, there is no capability to both detect and jam inthe same frequency range.

In a detection system (e.g., system 101) that determines an optimum scanstrategy, it may be beneficial to take into account operation of ajammer, and determine an alternate scan strategy accordingly. It isrealized that when a jammer is on, the jammer signal affects thefrequency band in which it transmits, and adjacent bands in whichharmonics are produced by the jammer signal. Also, it is realized thatfor a jammer to be effective, it should be operating as continuously aspossible, so that it can reduce the possibility that the vehicle inwhich the detection system exists cannot be detected by a threat thatproduces an emitter signal to be “jammed.” Thus, it is a goal tominimize the amount of time the jammer is off. However, this goalcompetes with the need for a receiver to operate in the frequency bandsaffected by the jammer signal, such that the detection system can detectthe threat. Therefore, a detection system is provided to balance theneeds of an active jammer to operate effectively, yet allow a receiverto operate within a band affected by the jammer signal. According to oneembodiment, scan strategies are determined for both jamming and non jamsituations.

To provide this capability, the detection system may be provided severaladditional inputs in addition to the emitters to be detected. Inparticular, this input information defines which bands are affected bythe operation of one or more active jammers. For example, additionalinputs to be used in performing this task include a Jammer BlankingTable, and a Receiver Blanking Table as discussed in more detail below.The capability to generate “dry” (no jam) and “wet” (jamming) scanstrategies for an emitter set may be supported, with separate interceptrules for each.

A detection system may be switched between “Normal” and “Jammer” modes.When “Jammer” is selected, the following processing changes may beperformed:

Information Matrix Computation Changes:

-   -   One of the six multiple intercept rules of the database is        interpreted as containing jamming mode intercept rules (referred        to hereinafter as “Jamming Mode Intercept Rules”) that determine        how emitters affected by the active jammer should be processed        when jamming is enabled. By evaluating these Jamming Mode        Intercept Rules, a scan strategy different from the “dry” (no        jam) strategy using the same emitters may be determined.    -   A subset of the emitters in the data is identified as the        signals to be targeted by the jammer(s). This, for example, may        be operator input. This subset has corresponding Revisit Times        (RVT) computed using the Jamming Mode Intercept rules.    -   When establishing minimum Dwell Duration (MDT), for an emitter,        a fraction of the maximum Pulse Repetition Interval (PRI) may be        assigned for jammer candidates, while the max PRI is assigned to        the remaining emitters. A lower bound can be imposed to ensure        that MDT is not too small. The goal of a small MDT is to deny        the jamming candidates consecutive pulses when the jamming is        dropped for a receiver “look”.    -   To compensate for the fractional MDT's affect on Intercept        Performance, the RVT of the jammer candidates may be scaled by        MDT÷maxPRI, with the ratio limited to 1.0.

These changes allow for computation of dwell parameters for jammercandidates separately, assign a sub-PRI Dwell Duration, and stillmaintain a probability of intercept consistent with the Jammer ModeRules.

Extract Scan Strategy Changes:

-   -   When in Jammer mode, a Receiver Blanking Table and a Jammer        Blanking Table are processed. These tables map the frequency        ranges to band index numbers, and define which bands are        simultaneously affected by a jammer active in a single band. Two        tables are used because the jamming system (e.g., jammer        processor 207) and receiving system (e.g., receiver processor        206) may have different frequency band definitions (e.g., the        effects of signals in adjacent bands (coupling) may be different        between the two systems).    -   When processing a frequency range, the emitters within the range        are compared to the Receiver Blanking Table. The jamming        candidates are identified for the range. If any emitters are        present within the range, or if the Receiver Blanking Table        indicates that there adjacent candidates that affect the range,        then the minimum MDT (referred to as “JAMMDT”) of all the        related candidates is returned and is used as the MDT solution        of the frequency range, replacing the MDT of each emitter.        Similarly, the RVT of each updated emitter is scaled by JAMMDT        divided by the original MDT. Once the dwells are built, this        allows the jamming in the common bands to be paused for the        common minimum time while the receiver “looks” for the        signal(s).    -   With the input data now conditioned for the affects of the        jamming candidates, the optimum set of dwells may be built.    -   The Jammer Blanking Table may be used to map each dwell affected        by the jamming to the jammer bands to exert “look” control. This        allows the receiver to blank multiple jammer bands for the        dwell's “look” time.    -   If a receiver frequency range is found to be free of a jammer        assignment, then its dwells are built and assigned normally.

In this manner, the detection system may take into account the operationof active Jammers when computing a scan strategy.

Dwell Placement

As mentioned above, once the information matrix including the emittersof interest is computed, a scan strategy may be extracted from theinformation matrix. The scan strategy is defined by one or more dwellsand describes how the receiver's resources may be utilized. It isdesirable for the scan strategy to use these resources efficiently andthe parameters of the dwells which define the scan strategy directlyimpact the efficiency of the scan strategy. A dwell is defined byseveral parameters. These parameters include the frequency range whichthe dwell is intended to cover, the dwell duration (i.e., the amount oftime the receiver spends tuned to that frequency range), and a revisittime (i.e., how often the dwell is executed).

Thus, for example, when executing a dwell having a frequency range of1100 MHz-1200 MHz, a dwell duration of 10 milliseconds (ms), and arevisit time of 125 ms, the receiver will spend 10 ms tuned between 1100MHz and 1200 MHz. Revisit time is measured from the beginning of thefirst execution of the dwell to the beginning of the next execution ofthe dwell. After the dwell has completed execution (i.e., after the 10ms dwell duration has expired), 115 ms will elapse before the dwell isexecuted again. An additional dwell parameter is the detecting method.The detecting method of a dwell is the IF and VBW filter bandwidthcombination. Each detecting method has an associated sensitivity. Thedetecting method affects the frequency range of a dwell, as the dwell'smaximum frequency range is limited by the bandwidth of the receiver's IFfilter. The revisit time of the dwell is also dependent on the detectingmethod as a more sensitive detecting method will yield a higherprobability of detection. Thus, the more sensitive (i.e., smallerbandwidth) detecting method used, the greater the revisit time will be.

An example of a simple scan strategy for detecting two emitters is shownin FIG. 11. The first emitter, emitter 1, operates in the frequencyrange of 1000-1200 MHz while the second emitter, emitter 2, operates inthe frequency range of 1300-1400 MHz. For the purposes of this example,each dwell is assumed to use the same detecting method and the IF filterof the detecting method is assumed to have a bandwidth of 50 MHz. Thus,the frequency range of each dwell is limited to 50 MHz. The scanstrategy includes eight dwells. Dwell 1 has a frequency range of1000-1050 MHz. Dwell 2 has a frequency range of 1050-1100 MHz. Dwell 3has a frequency range of 1100-1150 MHz. Dwell 4 has a frequency of rangeof 1150-1200 MHz. Dwell 5 has a frequency range of 1200-1250 MHz. Dwell6 has a frequency range of 1250-1300 MHz. Dwell 7 has a frequency rangeof 1300-1350 MHz. Dwell 8 has a frequency range of 1350-1400 MHz. Eachof the eight dwells has a duration of 25 ms. Because the eight dwellsare executed in succession and the process of executing all eight dwellsrepeats after Dwell 8 is executed, the revisit time of each dwell is 200ms (8×25). Thus, this scan strategy has the effect of sequentiallystepping through the frequency spectrum from the minimum frequency ofthe lowest frequency emitter (i.e., emitter 1) to the maximum frequencyof the highest frequency emitter (i.e., emitter 2).

The pulse repetition interval (PRI) of a signal is the time betweenpulses in the signal of an emitter. As shown in FIG. 11, the PRI ofEmitter 1 is 20 ms and the PRI of Emitter 2 is 5 ms. Typically, theminimum dwell duration of a dwell is set to the highest PRI of theemitters that the dwell is intended to detect. This way, the executionof a dwell will not fall in between pulses in the emitters' signals. Inthe example of FIG. 11, because the highest PRI of emitters is 20 ms,(i.e., the PRI of Emitter 1) the minimum dwell duration of the dwells inthe scan strategy should be at least 20 ms.

While the scan strategy of FIG. 11 is effective, in that it is capableof detecting both Emitter 1 and Emitter 2, it is not an efficient use ofthe receiver's resources. Because here is no emitter to be detected thatuses the 1200-1300 MHz range, no emitter will be detected by Dwell 5 andDwell 6. These two emitters illustrate a problem associated withsequentially scanning the frequency spectrum. This problem is that thereceiver spends time scanning a portion of the frequency spectrum inwhich no emitters of interest are operating. Thus, the receiver iswasting time scanning this portion of the spectrum that could be used toscan portions of the frequency spectrum in which emitters of interestare operating. Indeed, Dwell 5 and Dwell 6 could both be removed fromthe scan strategy without reducing the likelihood of detecting signalsfrom one of the emitters.

Dwell 7 and Dwell 8 cover the 1300 MHz-1400 MHz portion of the frequencyspectrum. These dwells cover the portion of the frequency spectrum inwhich Emitter 2 operates. Removing these dwells from the frequencyspectrum may result in failure to detect signals from Emitter 2.However, as discussed above, the dwell duration of both Dwell 7 andDwell 8 is 25 ms, while the PRI of Emitter 2 is 5 ms. Executing a 25 msdwell to detect an emitter signal with a PRI of 5 ms may waste receiverresources as it may not be necessary to wait for 25 ms to detect a pulsefrom Emitter 2. That is, if the dwell duration of Dwells 7 and 8 may bereduced to 5 ms, without the dwell being executed between pulses of thesignal from Emitter 2. This concept is illustrated more fully in FIG.12.

In FIG. 12, a signal from Emitter 3101 is shown having a PRI of 20 ms.Solution 1 shows a dwell in the frequency range of Emitter 3101 having adwell duration of 10 ms and a revisit time of 40 ms. The dwell isinitially executed at 5 ms. However, because the duration is only 10 ms,dwell execution is completed before the next pulse in the signal ofEmitter 3101. Thus, execution of the dwell falls in between Pulse 1 andPulse 2 of the signal of Emitter 3101. The revisit time of the dwell is40 ms, thus the dwell is “revisited” and again executed at 45 ms. Thistime the dwell falls in between Pulse 3 and Pulse 4 of the signal ofEmitter 3101. As can be seen, unless the timing of execution of thedwell happens, by chance, to line up with the timing of the pulses ofthe signal from the emitter, then it is possible for a dwell, which isotherwise capable of detecting the emitter signal, to fall in betweenpulses of the signal and consequently fail to detect the emitter signal.This problem can be solved by increasing the dwell duration. Forexample, if the minimum dwell duration is as long or longer than the PRIof the emitter signal, then the dwell does not fall in between pulses ofthe signal.

Solution 2 of FIG. 12 shows an alternate solution where the dwellcovering the frequency range of Emitter 3101 has a duration of 20 ms(i.e., the PRI of Emitter 3101) and a revisit time of 40 ms. The dwellis first executed at 5 ms, however unlike solution 1, execution of thedwell is not complete until 25 ms, thus Pulse 2, which occurs at 20 ms,is detected, as indicated by the asterisk in Solution 2. After 50 ms,the dwell is “revisited” and again executed at 55 ms. This time, Pulse 4is detected by the dwell, as indicted by the asterisk at 60 ms. As canbe seen in the example of FIG. 12, by increasing the minimum dwellduration to the maximum possible PRI of the emitter, execution of thedwell does not slip in between pulse of the emitter signal. In Solution2 of FIG. 12, the dwell duration remains the same whether or not a pulseis detected. As will be discussed later in greater detail, it should beappreciated that dwell duration may be extended if a pulse is detectedduring that dwell (e.g., to verify the presence and othercharacteristics of the emitter signal). It should further be understoodthat there are, however, certain situations in which the duration of adwell may be lower than the maximum possible of PRI of the emitter whichthe dwell covers. Such a situation may occur, for example, when therevisit time of the dwell is short enough that the dwell is adequatelyover-sampled, such that desired probability of detection is notsacrificed. Such a situation will be discussed in detail below.

Thus, in the example of FIG. 11, it can be seen that the efficiency ofthe scan strategy may be improved by eliminating Dwells 5 and 6 whichcover a portion of the frequency spectrum unused by any of the emittersof interest. The efficiency can further be improved by reducing thedwell duration of Dwells 7 and 8 to the maximum possible PRI of Emitter2. Additionally, each dwell in the scan strategy of FIG. 11 uses thesame detecting method. As discussed above, by using a differentdetecting method, the frequency range of the dwell as well as therevisit time of the dwell may be altered. For example, if a moresensitive detecting method were used to create the scan strategy, thefrequency range of each dwell might be decreased, requiring a greaternumber of dwells to cover the entire frequency range of all emitters ofinterest (assuming that each dwell uses the same detecting method).Intuitively, an increased number of dwells results in an increased cost,in terms of the receiver's resources. However, a higher sensitivitydetecting method may also result in an increased revisit time. Theincreased revisit time reduces the cost of executing a dwell andtherefore may offset the cost of the additional dwells and reduce theoverall cost of the scan strategy.

For example, consider a simplified information matrix 4000 of FIG. 13.The first column of information matrix shows that there are fouremitters of interest: E1, E2, E3, and E4. Each row of information matrix4000 contains data about one of the emitters. The second and thirdcolumns of the information matrix provide information about the revisittime for a particular detecting method. For example, the second columnprovides revisit time information for the detecting method of 250 MHzIF/15 MHz VBW. The third column provides revisit time information forthe detecting method 30 MHz IF/15 MHz VBW. For the purposes ofsimplicity in this example, only two detecting methods are show ininformation matrix 4000. However, it should be appreciated that anyreasonable number of detecting methods may be provided in theinformation matrix. Likewise, although there are only four emitters ofinterest shown in information matrix 4000, any number of emitters ofinterest, such as one, two, three, or five or more emitters may beprovided in the information matrix. It should also be understood thatinformation matrix 4000 has been simplified for the purposes of thisexample and the information matrix may include more information than isshown in FIG. 13.

In addition to including additional detecting methods and emitters ofinterest, the information matrix may also include multiple pulse sortsets that contain information used to condition the pulse train forsignal processing. Because these pulse sort sets affect dwell duration,there may be multiple pairs of minimum dwell duration time (MDT) valuesand extended dwell duration time (EDT) values. As discussed above, theMDT value is typically the maximum possible PRI of the emitter. If apulse is detected during a dwell, the dwell duration may be extended tocollect additional pulses. The more pulses that are collected, thelonger the EDT of the dwell. The number of pulses that are collected isdependent on the information in the pulse sort set. Thus, depending onwhich pulse sort set is used the EDT of the dwell will vary. Thus, foreach pulse sort set that is provided in the information matrix, they mayalso be a corresponding MDT/EDT pair.

Returning to the example of FIG. 13, the third and fourth columnsindicate the frequency range in which the emitter operates. For example,emitter E1 operates between 1000-1300 MHz and emitter E2 operatesbetween 1220-1350 MHz. The last column of information matrix 4000 is theMDT of E1. As mentioned above, the information matrix typically containsan MDT/EDT pair. As will be discussed below in greater detail, the costof executing a dwell is based, in part, upon the duration of that dwell.The actual duration of the dwell may be, for example, MDT or EDT,depending on whether a pulse was detected during execution of the dwell.Thus, one may estimate the actual dwell duration by assuming that acertain number of MDT dwells and EDT dwells will occur. However, for thesake of simplicity, in this example the cost of the dwell will becomputed using only the MDT of the dwell.

In one embodiment of the invention, the cost to the receiver ofexecuting a dwell is measured by the ratio of dwell duration to revisittime. The cost may be computed by the equation in Table 3. In theequation of Table 3, c represents the cost to the receiver, DD_(MAX)represents the highest dwell duration of all emitters covered by thedwell, and RVT_(MIN) represent the shortest revisit time of all emitterscovered by the dwell.

TABLE 3 $c = \frac{{DD}_{MAX}}{{RVT}_{MIN}}$

For example, suppose that a dwell using Detecting Method 1, 250 MHzIF/15 MHz VBW, covered both emitters E1 and E2 of information matrix4000. The dwell duration of emitter E1 is 3 ms, while the dwell durationof emitter E2 is 5 ms. Because emitter E2 has a higher dwell duration,the dwell duration of emitter E2 will be used in computing the cost.However, because the revisit time of emitter E1 (100 ms) is shorter thanthe revisit time of emitter E2 (120 ms), the revisit time of emitter E1will be used in the cost computation, as shown in Table 4. Thus, thecost of a dwell that covers both emitters E1 and E2 is 0.05 or 5%.

TABLE 4 DD_(MAX) = 5 RVT_(MIN) = 100 $c = {\frac{5}{100} = {.05}}$

Using a more sensitive detecting method (e.g., Detecting Method 2) maydecrease the cost of the dwell (i.e., by increasing the value ofRVT_(MIN). However, using a more sensitive detecting method may requirethe use of more dwells to cover the same portion of the frequencyspectrum, due to the decreased bandwidths of the IF and VBW filters.Thus, it is desirable to reduce the cost of a dwell by increasing thesensitivity of the detecting method as much as possible withoutincreasing the number of dwells to the point that the cost of theadditional dwells outweighs the cost savings of using the more sensitivedetecting method. Consider, as an example, constructing a scan strategyfor emitters of information matrix 4000. When one or more dwells havebeen constructed as part of the scan strategy that cover an emitter,that emitter is removed from the information matrix. Thus, as the scanstrategy is constructed, the number of emitters in the informationmatrix will decrease. First, using the first detecting method ininformation matrix 4000 (i.e., 250 MHz IF/15 MHz VBW detecting method),a dwell is constructed for emitter E1. The dwell starts at the minimumfrequency of emitter E1's frequency range (i.e., 1000 MHz) and extendsupwards to the detecting method's maximum frequency. Thus, as shown inSolution 1 of FIG. 14A, which utilizes Detecting Method 1, Dwell 1 iscreated which covers the portion of the frequency spectrum from1000-1250 MHz. Because the frequency range of Emitter E2 overlaps thatof Emitter E1, part of the frequency range Emitter 2 is also covered byDwell 1. Thus, this dwell may be used to cover part of the frequencyrange of Emitter E1 and Emitter E2. As a result, the cost of Dwell 1 isthe ratio of the maximum dwell duration between Emitter E1 and EmitterE2 to the minimum revisit time of those two emitters. Thus, as shown inFIG. 14A, the cost of this dwell is 5/100 or 0.05.

Solution 2 illustrates the cost of covering the same frequency rangewith a higher sensitivity detecting method. In Solution 2, DetectingMethod 2 (i.e., 30 MHz IF/15 MHz VBW) is used to cover the portion ofthe frequency spectrum ranging from 1000 MHz to 1270 MHz. Because of thesmaller bandwidth of this detecting method, more dwells are needed tocover the same portion of the frequency spectrum. However, because therevisit time associated with the more sensitive detecting method isgreater than that of the less sensitive detecting method, the cost perdwell is reduced. In Solution 2, Dwells 1-7 cover only Emitter E1because Emitter E2 does not operate in the frequency range covered byDwells 1-7. Thus, Dwells 1-7 have a DD_(MAX) of 3 ms and a RVT_(MIN) of650 ms. As a result, the cost of each of Dwells 1-7 is 3/650. However,Dwells 8 and 9 cover both Emitter E1 and Emitter E2, thus the DD_(MAX)of these dwells is 5 (i.e., the MDT of Emitter E2) and the RVT_(MIN) is650 ms (i.e., the RVT of Emitter E1). Thus, the cost of each of Dwells 8and 9 is 5/650. As shown in Table 5, the total cost is 31/650 orapproximately 0.048.

TABLE 5${{7\left( \frac{3}{650} \right)} + {2\left( \frac{5}{650} \right)}} = {\frac{31}{650} \approx {.048}}$

Thus, in the example of FIG. 14A, the more sensitive detecting methodyields a lower total cost. However, in some situations the lesssensitive detecting method yields a lower total cost. For example,Solution 1 of FIG. 14B shows a dwell constructed to cover part ofEmitter E3 of FIG. 13 using Detecting Method 1. This dwell, Dwell 1 ofSolution 1, ranges from the minimum frequency of Emitter E3 (i.e., 1510MHz) to the maximum frequency of the detecting method (i.e., 1760 MHz).Dwell 1 also covers part of the frequency spectrum in which Emitter E4operates, thus the dwell durations and revisit times of both Emitters E3and E4 may be taken into account when determining the cost of the dwell.As a result, DD_(MAX) is 4 ms and RVT_(MIN) is 330 ms, yielding a dwellcost of approximately 0.012 for Dwell 1.

Solution 2 covers the same portion of the frequency spectrum using themore sensitive detecting method, Detecting Method 2. In this case, nineDetecting Method 2 dwells are needed to cover the same portion of thefrequency spectrum as one Detecting Method 1 dwell. In Solution 2,Dwells 1-7 cover only Emitter E3, as Emitter E4 does not operate in theportion of the frequency spectrum covered by Dwells 1-7 (i.e., 1510MHz-1720 MHz). Thus, the cost of each of Dwells 1-7 is computed using aDD_(MAX) of 2 ms and an RVT_(MIN) of 330 ms (i.e., the minimum dwellduration and revisit time of Emitter E3). However, Dwells 8 and 9 coverportions of the frequency spectrum that may be used by both Emitters E3and E4. Thus, the cost of each of Dwells 8 and 9 is computed using aDD_(MAX) of 4 ms (i.e., the dwell duration of Emitter E4) and anRVT_(MIN) of 330 ms (i.e., the revisit time of Emitter E3). As shown inTable 6, the total cost of these nine dwells is approximately 0.06. Inthis case, a single lower sensitivity dwell (i.e., Solution 1) yields alower cost than multiple higher sensitivity dwells (i.e., Solution 2).

TABLE 6${{7\left( \frac{2}{330} \right)} + {2\left( \frac{4}{330} \right)}} = {\frac{22}{330} \approx {{.0}\overset{\_}{6}}}$

Therefore, when extracting a scan strategy from the information matrix,it is desirable to determine which detecting method yields the lowestcost for a particular dwell or set of dwells. It should be appreciatedthat in the example shown in FIGS. 14A and 14B the dwells constructed(in both Solution 1 and Solution 2) did not cover the entire frequencyrange in which each of the emitters, E1, E2, E3, and E4 operates. Tocompletely cover the entire frequency range of each of these emitters,it may be necessary to construct additional dwells. In one embodiment ofthe invention, the information matrix is updated based on what portionof the frequency spectrum of an emitter in the matrix has been coveredby a dwell. When the information matrix is empty, then all emitters havebeen completely covered by dwells and the scan strategy is complete.

For example, in FIG. 14A, assume that the scan strategy uses Solution 2(i.e., the lower cost solution) and Dwells 1-9 of Solution 2 areconstructed. Information Matrix 4000 of FIG. 13 may be updated asfollows. The RF Min values for Emitter E1 may be changed to 1270 MHz,because the 1000-1270 MHz range is covered by Dwells 1-9. Similarly, theRF Min value of Emitter E2 may be changed 1270 MHz because the 1220-1270MHz range is been covered by Dwells 8 and 9. When additional dwells areconstructed to cover the remaining portion of the frequency range ofthese two emitters, these two emitters may be removed from InformationMatrix 4000.

The examples in FIGS. 14A and 14B compute the cost of a dwell under theassumption that dwell duration will always be MDT_(MAX), that is, thedwell duration will be the maximum PRI of the emitters covered by thatdwell. However, as mentioned above, in certain situations the actualdwell duration may be longer than MDT_(MAX). These situations may occur,for example, where a pulse is detected during a dwell. If a pulse isdetected, the dwell duration may be extended based on the computed EDT.Thus, when a pulse is detected during a dwell, the cost of that dwellmay be increased if the dwell duration is extended from MDT to EDT.Thus, it may be desirable to take into account the occurrence of someEDT dwells when computing cost while extracting the scan strategy. Inone embodiment of the invention, a steady state model may be used, whereit is assumed that a certain number of EDT dwells and a certain numberof MDT dwells will be executed over a specific period of time. If thisspecific period of time is called exam_time, then Table 7 shows anequation for estimating the number of MDT dwells and EDT dwells thatwill occur in that period of time. The number of MDT dwells executed forevery EDT dwell. The variable RVT represents the revisit time of thedwell.

TABLE 7 ${Count} = \left\lbrack \frac{Exam\_ Time}{RVT} \right\rbrack$

In the equation of Table 7, Count is defined as Exam_Time divided by therevisit time of the dwell, RVT. The value of Exam_Time may be selected,for example, based on the signal processing algorithms used and theoverall affect of the signal environment, based on field tests. OnceCount has been determined by the equation in Table 7, an Actual DwellDuration may be expressed as a weighted average of MDT and EDT, as shownin the equation of Table 8.

TABLE 8 ${{Actual\_ Dwell}{\_ Duration}} = \frac{\begin{matrix}{\left( {{TO} + {EDT}} \right) + {\left( {{Count} - 1} \right) \times}} \\{K \times \left( {{TO} + {MDT}} \right)}\end{matrix}}{Count}$

The equation of Table 8 computes this weighted average, assuming sometuning overhead (TO), or dead time between dwells, as a result of tuningthe receiver. The constant K, in the equation, accounts for multiplefields of view per dwell cycle. It should be appreciated that theequations of Table 7 and Table 8 are merely an example of method forestimating actual dwell duration to determine dwell cost. Many othermethods for estimating actual dwell duration may be used and areintended to be within the spirit and scope of the invention.

In the example of FIG. 14A, as mentioned above, the first dwell or setof dwells was created starting with the lowest RF Min value inInformation Matrix 4000 of FIG. 13. However, if the lowest RF Min valuein the information Matrix is used to construct the initial dwell, otherpossible scan strategies, which may or may not yield a lower cost, maynot be considered. For example, FIG. 17 shows two emitters, E1 and E2.Emitter E1 has an RF Min value of 1100 MHz and an RF Max value of 1200MHz. Emitter E2 has an RF Min value of 1150 MHz and an RF Max value of1250 MHz. Suppose the initial dwell is constructed using the lowestvalue of RF Min, when constructing a scan strategy for Emitters E1 andE2. Scan strategy 7001 is one possible scan strategy that may resultfrom using the lowest value of RF Min to construct the initial dwell. Inthis example, Dwell 1, which uses detecting method M1, covers the1100-1200 MHz range. The remaining portion of the frequency spectrum inwhich emitter E2 may operate is covered by Dwell 2 and Dwell 3 whichuses a greater sensitivity detecting method, M2. Scan strategy 7003,which might yield a lower cost than scan strategy 7001, depending ondwell parameters, would not be considered if the lowest value RF Min(1100 MHz) was initially used. When initially using the lowest value RFMin, even if it had been decided to use the greater sensitivity method,M2, at the bottom of the frequency spectrum, four M2 dwells would havebeen constructed. Then, the remaining portion of the frequency spectrumused by emitter E2 would have been covered by additional dwells (e.g.,one M1 dwell or two M2 dwells).

However, by altering the frequency at which the initial dwell isconstructed, other possible scan strategies may be constructed. Forexample, suppose that the initial dwell is constructed using the RF Minof emitter E2 (i.e., 1150 MHz). A scan strategy such as scan strategy7003 may result. In scan strategy 7003, the initial dwell, Dwell 1, usesdetecting method M1 and covers the 1150-1250 MHz range. Because theinformation matrix would not be empty after the construction of Dwell 1,Dwells 2 and 3 may be constructed to cover the portion of the frequencyspectrum in which emitter E1 operates, but which is not covered by Dwell1. Thus, by varying the RF Min at which the initial dwell isconstructed, different lower cost scan strategies may result.

A flow chart for constructing scan strategies with varying initial RFMin values, according to one embodiment of the invention, is shown inFIG. 15. At act 5001, a Limit Vector is created. The Limit Vector is avector of each of the RF Min values in the information matrix. Thus, forthe two emitters, E1 and E2 in FIG. 17, the Limit Vector would be, forexample, [1100 1150], because the RF Min value for emitter E1 is 1100and the RF Min value for emitter E2 is 1150. Also at act 5001, avariable n is initialized to 1. The variable n represents the currentposition in the Limit Vector on which the process is operating. Thus,initially the process operates on the first value in the Limit Vector.

The process then continues to act 5002, where a variable LIMIT isdefined as the nth element in the Limit Vector. Because n initially hasthe value 1, LIMIT is first set as the first element in the LimitVector. Using the example of FIG. 17, with emitters E1 and E2, LIMITwould first be set to 1100. The process then continues to act 5003,where the scan strategy is extracted using the value of LIMIT. As willbe discussed in greater detail below, extracting the scan strategyincludes building dwells to cover the emitters in the information matrixand evaluating different the cost of using different detecting methodsfor these dwells.

Next the process continues to act 5004 where the cost of the scanstrategy is compared to the cost of BEST, which is the scan strategywhich has the lowest cost so far. BEST is initialized to a scan strategyhaving infinite cost, so the first scan strategy extracted at act 5003will be lower in cost than BEST. If the cost of the extracted scanstrategy is lower than the cost BEST, then the extracted scan strategyis saved as BEST and the process continues to act 5006. If the extractedscan strategy is not lower in cost than BEST, then act 5005 is skippedand the process continues directly to act 5006. At act 5006 it isdetermined if LIMIT is the last element in the Limit Vector. If LIMIT isthe last element in the Limit Vector, then the scan strategy BEST isreturned as the scan strategy to be used for the emitters in theinformation matrix. If LIMIT is not the last element in the LimitVector, the value of n is incremented by one, and the process returns toact 5002. At act 5002 LIMIT is redefined as the next element in theLimit Vector and the process repeats using this new LIMIT value.

In the example of FIGS. 14A and 14B, the initial detecting method usedto create a dwell was the lowest sensitivity, widest bandwidth,detecting method. Then, the greater sensitivity detecting method wasevaluated to see if it yielded a lower cost for a dwell or set of dwellshaving a bandwidth defined by the lowest sensitivity detecting method.Thus, the number of dwells needed for the greater sensitivity detectingmethod was based on this initial bandwidth of the lowest sensitivitydetecting method. It has been recognized that in some situations, thecost of a scan strategy may be reduced if the initial bandwidth isvaried from that of the lowest sensitivity detecting method. The costmay be reduced because each detecting method may differ in instantaneousfrequency coverage and sensitivity, which in turn alters the mix ofemitters in the database which may be satisfied by a dwell having afrequency range based on the bandwidth of the detecting method. That is,if one were to assume that the lowest sensitivity detecting method wasnot available, then a lower cost scan strategy may result from using thebandwidth of the next greatest sensitivity detecting method as theinitial bandwidth. That is, the method for extracting a scan strategymay loop on all available detecting methods, progressively inhibitingdetecting methods. This may be done for each value of the Limit Vector.That is, each time the process in FIG. 15 extracts a scan strategy atact 5003 (i.e., with a different value for LIMIT), the loop on detectingmethods may be performed. As a result, a total of M×N scan strategiesmay be generated, where M represents the number of elements in theMethod Vector and N represent the number of elements in the LimitVector.

A process for looping on detecting methods, according to one embodimentof the invention, is shown in FIG. 18. At Act 8001 a Method Vector iscreated. The Method Vector is a vector of all detecting methods, orderedfrom lowest sensitivity to greatest sensitivity. The variable m whichrepresents the current position in the Method Vector is initialized toone. The variable LOWEST which represents the scan strategy with thelowest cost so far is initialized to a scan strategy having a cost ofinfinity. The process then continues to act 8003, where the variableMETHOD is defined as the m^(th) value in the Method Vector. As m isinitialized to one, METHOD will initially be the first value in theMethod Vector. At act 8005, the Create Dwell Set Process is invoked tocreate the dwell set using METHOD as the initial detecting method thatdefines the initial bandwidth. As will be discussed in greater detailbelow, The Create Dwell Set Process also uses the current value of LIMITas determined in the flow chart of FIG. 15. If it is decided not to loopon LIMIT values (e.g., to omit the process of FIG. 15), then CreateDwell Set Process may simply uses the lowest RF Min value in theinformation matrix instead of LIMIT. As will also be discussed below,when the Create Dwell Set Process evaluates various detecting methods,the initial dwells will be created using the detecting method specifiedby METHOD. The cost of using METHOD will then be compared to all ofusing all the greater sensitivity detecting methods to cover thatportion of the frequency spectrum.

After the dwell set is created, the process continues to act 8007, wherethe cost of the created dwell set is compared to the cost of LOWEST. Ifthe cost of the created dwell set is less than the cost of lowest, theprocess continues to act 8009 where the created dwell set is saved asLOWEST. It should be appreciated that the first time a dwell set iscreated (i.e., before the process loops back at act 8013), the createddwell set will have a lower cost than LOWEST, as LOWEST was initializedto a dwell set having infinite cost.

If the created dwell set does not have a lower cost than LOWEST, theprocess continues directly to act 8011. At act 8011 it is determined ifMETHOD is the last detecting method in the Method Vector. If so, then atact 8015 the scan strategy LOWEST is returned. Otherwise, m isincremented by one, and the process returns to act 8003, and repeatedusing the next value in the Method Vector.

As mentioned above, when creating a dwell set at act 8005, it isdesirable to determine which detecting method yields the lowest cost fora dwell or set of dwells. FIG. 16 shows an example Create Dwell SetProcess for performing act 8005 according to one embodiment of theinvention. At act 6001 the variable MinFREQ is set to the current valueof LIMIT. The process continues to act 6003, where set D is defined asthe “core” emitters based on MinFREQ and METHOD.

The core emitters may be determined as follows. First, MaxFREQ maydefined as the maximum frequency of METHOD starting at MinFREQ andextending upwards. Then the set of emitters INTERNAL may be defined asall of the rows (i.e., emitter modes) in the information matrix thathave a frequency range (i.e., RF Min and RF Max) completely containedwithin MinFREQ and MaxFreq of METHOD. The set EXTERNAL may be defined asthe rows of the information matrix whose frequency contains the entirefrequency range of MinFreq to MaxFreq. That is, the frequency range ofMETHOD, starting at MinFREQ, is completely contained with the RF Min andRF Max of each emitter mode in the set EXTERNAL. Set D may be defined asthe union of the sets EXTERNAL and INTERNAL. The emitters in set Drepresent the “core” emitters.

Emitters may then be pre-filtered out of set D, if METHOD is a poorchoice for detecting these emitters. Each emitter mode has apre-filtering flag associated with each detecting method. Thus, forexample, if there are eight detecting methods, there would be eightpre-filtering flags for each row (i.e., each emitter mode) in theinformation matrix. If the pre-filtering flag for associated with aparticular detecting method is false, then the flag indicates that thedetecting method is a poor choice for solving for that emitter mode.

The flags may be set by computing the cost of each of the methods,multiplied by the number of dwells required to cover the row's frequencyrange. If there is no difference in cost, all methods for a row remainenabled. If there is a big difference in cost, then the method is acandidate to be filtered out via the flags. However, a detecting methodmay only be filtered out via the flags if the cost of other rows withinthe IF bandwidth of the method are not similar (or greater in cost) thanthe given method. By pre-filtering out some detecting methods, thealgorithm reduces the amount of computation necessary to find a highlycost effective solution by eliminating detecting methods which areunlikely to yield satisfactory solutions for particular emitters.

Each time a dwell is built, the flags may be recomputed. This may benecessary because the dwell may have removed adjacent rows from theinformation matrix or the frequency range of the row may have be reducedbecause the dwell that was just built covered part of the frequencyrange of that row. Reducing the frequency range of row alters the costof the detecting methods. Also, after a dwell is built, a check may bemade to see if any of the pre-filtered out rows are incidentally coveredby the dwell. If they are, then they may included in that dwell anywayand removed from the information matrix. Thus, pre-filtering ofdetecting methods may help set up the solution search, but does notnecessarily prevent a method from being used.

After pre-filtering has been applied to set D, removing any rows fromthe set D which are a poor choice for METHOD, the process continues toact 6005 where any “free” overlapping emitters may be added to set D. Atact 6005, sets HIGHOVERLAP and LOWOVERLAP may be defined. SetHIGHOVERLAP includes rows of the information matrix that have an RF Minvalue greater than MinFREQ and less than MaxFREQ, but have an RF Maxvalue greater than MaxFREQ. LOWOVERLAP includes rows of the informationmatrix that have an RF Max value greater than MinFREQ and less thanMaxFREQ, but have an RF Min value less than MinFREQ. The rows inHIGHOVERLAP and LOWOVERLAP may be termed “overlapping” emitters becausethe frequency range of these emitters overlaps the frequency range ofMETHOD, defined by MinFREQ and MaxFREQ. A row in one of these two setsmay be added to D if that row would not drive the dwell parameters(i.e., if the dwell duration is less than DD_(MAX) of the rows in D andrevisit time greater than the RVT_(MIN) of the rows in D).

However, a row in of these two sets may be added to D even if the rowwould drive the dwell parameters, as long as overall dwell count wouldnot increase. For example, if an emitter's frequency range overlaps withthe upper part of the frequency range of METHOD (i.e., an emitter inHIGHOVERLAP), and if this overlapping emitter's frequency range needstwo dwells to provide coverage of the entire frequency range, and theemitter's frequency range is overlapping METHOD's frequency range by atleast half of the emitter's frequency range, this emitter may be addedin to set D. As a result, only one additional dwell will be needed at alater time to cover the rest of the emitter's frequency range. However,if the overlap was only ten percent, there is no reason to include thisemitter in set D, because two dwells will still be generated at a latertime to cover the remaining ninety percent.

At this point, any emitters in set D which have conflicting hardwarecontrols may be removed from set D. That is, set D may be redefined asthe largest subset of set D which has no hardware controls conflicts.The process then continues to act 6007 where set DX is defined. DXincludes any emitters that are in the frequency range of D, but are notdetectable by detecting method METHOD (i.e., due to pre-filtering,hardware conflicts, etc.) After set DX is defined the process continuesto act 6009. At act 6009, it is determined which detecting method, ofgreater sensitivity than METHOD, yields the lowest cost for DX. Thisdetecting method may be called M2. Finding M2 may be accomplished byevaluating the cost of each detecting method for covering the desiredfrequency range, as in the examples of FIGS. 14A and 14B. It should beappreciated that, as in the examples of FIGS. 14A and 14B, the greatersensitivity detecting method may require more than one dwell to coverthe frequency range of the emitters in set DX. It should further beunderstood that in the examples of FIGS. 14A and 14B, for the sake ofsimplicity, only two detecting methods were evaluated to determine whichdetecting method yielded the lowest cost. However, many more detectingmethods could be used for a dwell. In one embodiment of the invention upto eight detecting methods are available, although any suitable numberof detecting methods could be used, as the invention is not limited inthis respect.

The process next continues to act 6011 where Cost 1 is defined as thecost of using METHOD as the detecting method for the emitters in set Dplus the cost of using detecting method M2 for the emitters in set DX.After Cost 1 is computed, the process continues to act 6012, where thedetecting method, of greater sensitivity than METHOD, that yields thelowest cost for the emitters in D is determined. This detecting methodmay be called M3. Similar to M2, detecting method M3 may be identifiedby evaluating the cost of dwell D for the available detecting methodsand selecting the one with the lowest cost. Because M3 is a greatersensitivity and a smaller bandwidth detecting method than M2, more thanone M3 dwell may be needed to cover the entire frequency range ofMETHOD. The process continues to act 6014, where Cost 2 is defined asthe cost of using M3 for the emitters in set D, plus the cost of M2 foremitters in DX. It should be appreciated that M3 and M2 may be the samedetecting method or may be different detecting methods. If M3 and M2 arethe same detecting method, it may be assumed that the emitters in DX andD are covered by the same set of M3 dwells, and Cost 2 is defined as thecost of these dwells. Otherwise, as mentioned above, Cost 2 is the sumof the cost of M2 dwells for DX and the cost of the M3 dwells for D.

Next, at act 6013, Cost 1 is compared to Cost 2. If Cost 1 is less thanCost 2, then a dwell is built for the emitters in D using METHOD. Adwell or multiple dwells are also built for the emitters in DX using M2.Otherwise, if Cost 2 is less than or equal to Cost 1, a dwell or set ofdwells is built for set D using method M3 and one or more dwells arebuilt for DX using M2.

It should be appreciated that MinFREQ may be altered as the process ofFIG. 16 is executed. Initially, MinFREQ is set to LIMIT. LIMIT is aproposed lower bound of a potential dwell. Thus, when the processstarts, it is testing the hypothesis that a dwell having the bandwidthof METHOD starting at LIMIT will satisfy the emitters in that frequencyrange. It is possible that when extracting the subset of emitters thathave compatible, non-conflicting hardware controls, the smallestfrequency of this set may be a frequency other than LIMIT. Anotherpossibility is that the emitter that defined LIMIT is the vastlydifferent in minimum frequency, relative to the other emitters that canbe covered, and it has been tossed out of the “core” set, D. In thiscase, D contains the emitters that we want to solve on this iteration,and its possible that the dwell start for this set is something otherthan LIMIT. In this case MinFREQ would be set to the dwell start for setD.

Once the dwell or set of dwells is built, the information matrix may beupdated to remove rows that are completely covered by the dwells and toalter the frequency ranges of the rows that are partially covered by thedwell or set of dwells. MinFREQ is again to be the lowest RF Min left inthe information matrix that is greater than LIMIT. If no such RF Minexists, then MinFREQ may be set to the lowest RF Min left in theinformation matrix. The process of FIG. 16 may then be repeated startingat act 6003, and using the new value of MinFREQ. Once the informationmatrix is empty, then a scan strategy has been constructed for thisparticular LIMIT and METHOD combination.

It should be appreciated that looping on the Limit Vector and MethodVector (i.e., M×N looping) are not necessary in building a scanstrategy. For example, LIMIT may simply be fixed at the lowest RF Min inthe information matrix and METHOD may be fixed at the lowest sensitivitydetecting method. The Create Dwell Set Process (e.g., in FIG. 16) maysimply be called using these fixed values for LIMIT and METHOD (e.g.,without using the LIMIT looping in FIG. 15 and the METHOD looping ofFIG. 18). Alternatively, LIMIT looping may be used without METHODlooping. That is, the value of METHOD is fixed at the lowest sensitivitydetecting method, and the Create Dwell Set Process is called for eachvalue in the Limit Vector. Alternatively, METHOD looping may be usedwithout LIMIT looping. In this scenario, the value of LIMIT is fixed atthe lowest RF Min initially in the information matrix and the CreateDwell Set Process is called for each value in the Method Vector.

Various modifications to the algorithms discussed above for creatingdwells as part of a scan strategy are available and intended to bewithin the scope of the invention. Such modifications include, but arenot limited to, modifications to the method of computing cost,modifications to the pre-filtering algorithm and the like. An example ofsuch a modification involves computing dwell cost using a smaller dwellduration value than the maximum from among the signal parameters of theemitters of interest. This modification will be discussed below ingreater detail.

Non-Maximum Dwell Duration Selection

As mentioned above, the minimum duration of a dwell is typically themaximum PRI of the emitters that the dwell is intended to cover.However, in some situations it may be possible to reduce the minimumdwell duration of a dwell to a length of time less than that of themaximum PRI of the emitters that the dwell is intended to cover. It maybe possible to reduce the minimum dwell duration, for example, when theoverall probability of intercept of a particular dwell may still be metwith a shorter dwell duration.

A first aspect of overall probability of intercept is the probabilitythat, during an illumination period, a pulse of the emitter will occurduring execution of dwell. As mentioned above, because the minimum dwellduration for detection of an emitter is typically set to the PRI of thatemitter, the probability of intercept is typically 1.0, because in mostcases, a dwell will not fall between pulses of the emitter. However, ifthe dwell is not executed during an illumination period, detection ofthe emitters may not occur. Thus, a second aspect of probability ofintercept is the probability that the dwell will be revisited during anillumination period. If the revisit time is “out of phase” to theillumination periods, then detection may not occur until after manyillumination periods. Worse, if the revisit time and illumination periodare exactly harmonically related and out of phase, then an intercept maynever occur. However, as the revisit time for a dwell is decreased, thelikelihood that the dwell will be revisited during an illumination timeis increased. Thus, as discussed above, a desired probability ofintercept is used in computing revisit times for emitters in theinformation matrix, for a particular detecting method. As a result, theoverall probability of intercept may be expressed as the probability ofexecuting the dwell during an illumination time (i.e., intercepting theemitter) multiplied by the probability of dwelling long enough tointercept sufficient energy to declare detection.

When a dwell is built from parameters of different emitters (e.g.,DD_(MAX) is associated with Emitter 1 and RVT_(MIN) is associated withEmitter 2), it may be possible to meet the overall probability ofintercept without considering the dwell duration of a particular emitterwhen determining the DD_(MAX) of the emitters. For example, suppose afirst emitter and a second emitter are covered by a dwell which has arevisit time associated with the second emitter (i.e., RVT_(MIN) is therevisit time of the second emitter). As a result, the overallprobability of intercept of the first emitter in that dwell hasincreased because the revisit time for the dwell is lower than therevisit time calculated for that emitter. A cost savings may beachievable by decreasing the overall probability of intercept for thatemitter (i.e., to bring the overall probability of intercept back toapproximately what was specified during revisit time calculation). Theoverall probability of intercept for the emitter may be decreased incertain situations by decreasing the dwell duration of the dwell to alength of time less than that of the PRI of the emitter. Thesesituations occur, for example, when the revisit time of the dwell (i.e.,the RVT_(MIN)) adequately over-samples for that emitter. It may bedetermined if the revisit time of the dwell adequately over-samples forthe emitter if the inequality in Table 9 is true. In Table 9, RVT_(MIN)represents the lowest revisit time of the emitters covered by the dwell.DD_(MIN) represents the dwell duration of the emitter having RVT_(MIN).Thus, DD_(MIN) and RVT_(MIN) are in the same row and are associated withthe same emitter. RVT_(N) and DD_(N) represent the revisit time anddwell duration of the emitter being tested to determine if DD_(N) may beexcluded when determining DD_(MAX) for the dwell.

TABLE 9${RVT}_{MIN} \leq {{RVT}_{N}\left( \frac{{DD}_{MIN}}{{DD}_{N}} \right)}$

Thus, if RVT_(MIN) is less than or equal to the product of RVT_(N) andthe ratio of DD_(MIN) to DD_(N), then RVT_(MIN) adequately over-samplessuch that DD_(MIN) may be used as the dwell duration without decreasingthe overall probability of detection desired for the emitter associatedwith DD_(N) and RVT_(N).

An example of such a situation is shown in FIG. 19. Emitter 1 in FIG. 19has a dwell duration of 1 ms and a revisit time of 500 ms. Emitter 2 hasa dwell duration of 2 ms and a revisit time of 1200 ms. Suppose thatwhen extracting a scan strategy for these two emitters (e.g., using themethod described above in connection with dwell placement), a dwell iscreated that covers both Emitter 1 and Emitter 2 of FIG. 19. Using themethod of computing cost as described in Table 3, the cost of this dwellwould be

$\frac{2}{500}$or 0.004, because the DD_(MAX) of the two emitters is 2 ms, and theRVT_(MIN) of the two emitters is 500 ms. However, as mentioned above,because the dwell covering Emitter 2 will now be executed every 500 msinstead of every 1200 ms, the overall probability of intercept ofEmitter 2 by the dwell has increased. As mentioned above, a cost savingsmay be achieved by decreasing DD_(MAX) to the dwell duration of Emitter1 (i.e., 1 ms), because the increased revisit time of the dwell,RVT_(MIN), adequately over-samples so that the original desiredprobability of intercept is not sacrificed.

For example, suppose that the desired probability of intercept specifiedwhen calculating the revisit time for Emitter 2 is 0.5. If theprobability of intercept based on dwell duration is 1.0 (i.e., if dwellduration is 2 ms), then the overall probability of intercept would be0.5. Also assume that if the revisit time for Emitter 2 were 500 ms,then the probability of intercept would be 1.0. Thus, when a dwell iscreated having a revisit time of 500 ms and a dwell duration of 2 ms,the overall probability of detection is 1.0. However, the overalldesired probability of intercept was previously specified as 0.5. Nowassume that decreasing the dwell duration from 2 ms to 1 ms decreasesthe probability of dwelling long enough to intercept sufficient energyto declare detection (i.e., the probability of intercept based on dwellduration) of Emitter 2 to 0.5. Now the overall probability of detectionEmitter 2 is back to 0.5 (i.e., the product of 1.0 and 0.5) and a costsavings is achieved by reducing the dwell duration from 2 ms to 1 ms. Asmentioned above, the inequality of Table 9 may be used as a test todetermine if an emitter is adequately over-sampled for by RVT_(MIN) sothat the dwell duration of that emitter may be disregarded indetermining the DD_(MAX) for the dwell. Table 10 shows the result ofthis inequality using the emitters of FIG. 19, where RVT_(MIN) andDD_(MIN) represent the revisit time and dwell duration of Emitter 1,respectively and RVT_(N) and DD_(N) represent the revisit time and dwellduration of Emitter 2, respectively.

TABLE 10 $\begin{matrix}{500 \leq {1200\left( \frac{1}{2} \right)}} \\{500 \leq 600}\end{matrix}\quad$

Because the inequality is true (i.e., 500 is less than or equal 600) forthe values of Emitter 1 and Emitter 2, the dwell duration of Emitter 2may be excluded when determining the DD_(MAX) for the dwell. In theexample of FIG. 19, the dwell would have a revisit time of 500 ms and adwell duration of 1 ms, yielding a cost of

$\frac{1}{500}$or 0.002.

In addition to cost savings in this manner, a cost savings may also beachieved in certain situations by running several dwells with differenttiming relationships, as opposed to a single dwell, even though thedwells otherwise have the same tuning configurations. As a result, thefrequency range of the set of emitters covered by the dwell or dwells isscanned by the receiver multiple times, but at different rates. Forexample, in FIG. 20, Emitter 1 and Emitter 2 have same hardware andtuning configuration and thus could be covered by a single dwell. Thecost of covering these two emitters with a single dwell would be 0.01(i.e., 5/500). The cost of covering these two emitters with two separatedwells (i.e., a first dwell for Emitter I and a second dwell for Emitter2) would be 0.007 (i.e., 0.002+0.005). In addition, every time thesecond dwell for Emitter 2 executes, it satisfies the detectionrequirements for Emitter 1 and Emitter 2, allowing the cost estimate of0.007 to be lowered.

For example, as illustrated in FIG. 21, Dwell 1, which covers Emitter 1has a revisit time of 500 ms, thus it is executed every 500 ms. Dwell 2has a revisit time of 1000 ms and is executed every 1000 ms. However,because Dwell 2 satisfies the detection requirements for Dwell 1 (i.e.,the dwell duration of Dwell 2 is greater than 1 ms), it may not benecessary to execute Dwell 1 at the 1000 ms intervals (i.e., 1000 ms,2000 ms, 3000 ms, etc.). As a result, the cost of executing Dwell 1 atthe 1000 ms intervals may be subtracted from the total cost of the twodwells. The equation in Table 11 is an example of an equation that maybe used to compute the cost of covering two emitters with separatedwells, taking into account the reduced cost provided by overlap of thedwells. In the equation of Table 11, DD₁ and RVT₁ represent the dwellduration and revisit time of the emitter with the shorter revisit time,while DD₂ and RVT₂ represent the dwell duration and revisit time of theemitter with the longer revisit time.

TABLE 11${Cost} = \frac{{DD}_{2} + {{DD}_{1}\left( {\frac{{RVT}_{2}}{{RVT}_{1}} - 1} \right)}}{{RVT}_{2}}$

Thus, in the example of FIG. 21, the cost of using independent dwells,as computed by the equation of Table 11, would be 0.006. Table 12 showsthis computation. Thus, using two independent dwells for Emitter 1 andEmitter 2 in FIG. 21 yields a cost of 0.006, as opposed to a cost of0.01 for a single or “merged” dwell.

TABLE 12 $\begin{matrix}{{Cost} = \frac{5 + {1\left( {\frac{1000}{500} - 1} \right)}}{1000}} \\{{Cost} = \frac{6}{1000}} \\{{Cost} = {.006}}\end{matrix}\quad$

FIG. 22 and FIG. 23 are flowcharts illustrating a method for determiningwhether the dwell duration of a dwell may be reduced from the DD_(MAX)of the emitters which the dwell is intended to cover and whether asingle dwell or two or more dwells with different timing relationshipsshould be used to cover these emitters. That is, the method of FIG. 22and FIG. 23 may reduce the cost of covering the emitters by returningthe number of dwells and the parameters of these dwells to be used inthe scan strategy. Thus, for example, when extracting the scan strategy,this method may be performed when computing the cost of a dwell.

At act 9000 of FIG. 22, the Data Array is created. Each row in the DataArray corresponds to an emitter that is covered by the dwell for whichthe cost is being computed. The Data Array has three columns. The firstcolumn is minimum dwell duration (MDT), the second column is extendeddwell duration (EDT), the third column is revisit time. An example of aData Array 9044 is shown in FIG. 24A. Data Array 9044 initially hasseven rows, indicating that the dwell is intended to cover sevenemitters. At act 9002, it is determined whether there is more than rowin the Data Array. If there is only one row in the Data Array, then thedwell only covers one emitter. Thus, the parameters of the dwell (i.e.,dwell duration and revisit time) which yield the lowest cost are simplythe dwell parameters associated with that emitter (e.g., the dwellduration and revisit time of the row in Data Array). Accordingly, theprocess continues to act 9004, where the dwell duration and revisit timeof the row in the Data Array are returned as the solution for theparameters of the dwell.

If, however, there is more than one row in the Data Array, the processcontinues to act 9006, where the rows are ordered by revisit time, fromthe shortest revisit time to the longest revisit time. If two rows havethe same revisit time, but different dwell durations, the row with theshorter minimum dwell duration may be replaced by the row with thelonger minimum dwell duration. In the example of Data Array 9044 of FIG.24A, because Data Array 9044 has more than one row, it is ordered byrevisit time, as shown in FIG. 24B. The process next continues to act9008, where any redundant rows and rows that are a subset of other rowsare removed from Data. A row is a subset of another row if it has agreater revisit time but a shorter MDT and EDT than the other row.Because this row will not drive the dwell parameters (i.e., its dwellduration will not be DD_(MAX) and its revisit time will not beRVT_(MIN)), it may be excluded from the cost analysis. In Data Array9044 of FIG. 24B, because row 4 9048 is a subset of another (e.g., row1) it may be removed. Additionally, because row 6 9050 is redundant withrespect to row 5 9049, it may also be removed. The resulting Data Array9044 is illustrated in FIG. 24C.

Once these rows are removed, the process continues to act 9010, whereeach row in the Data Array is compared to the first row in the DataArray to determine if that row's MDT may be excluded when determiningDD_(MAX) for the dwell. This may be done, for example, as describedabove using the equation of Table 9. That is, if the revisit time of thefirst row is less than or equal to the product of the revisit time ofthe row being compared and the ratio of the dwell duration of the firstrow to the dwell duration of the row being compared, then the row beingcompared passes the test and it's dwell duration may be excluded whendetermining DD_(MAX). In the example of Data Array 9044 in FIG. 24C, row4 9051 and row 5 9052 both pass this test. This indicates that a dwellmay be created having a DD_(MAX) of the first row and an RVT_(MIN) ofthe first row, which covers the emitters associated with the first row,row 4 9051, and row 5 9052. The process of FIG. 22 then continues to act9012, where the first row and any rows that were excluded in act 9010(e.g., any rows passing the test of the equation of Table 9) may beremoved from the Data Array. If no rows passed the test of the equationof Table 9, only the first row of the Data Array is removed.Additionally, a row is created in a Solution Array that indicates theparameters of a dwell which would cover the emitters associated with therows removed from the Data Array in act 9012. The parameters of thisdwell would typically be the DD_(MAX) and the RVT_(MIN) of the rowsremoved. However, because it was determined at act 9010 that the dwelldurations of the rows passing the test of the equation of Table 9 couldbe excluded when determining DD_(MAX), DD_(MAX) is set to the dwellduration of the first row. The EDT of the row added to the solutionmatrix may be set to the longest EDT of all the rows removed from theData Array. FIG. 24D shows Data Array 9044 and Solution Array 9046 afteracts 9010 and 9012 have been performed. As can be seen, the first row ofData Array 9044, as well as rows 9051 and 9052 have been removed fromthe Data Array. Additionally, a row has been created in Solution Array9046 which includes the parameters of dwell that would cover the rowsremoved from Data Array 9044.

The process of FIG. 22 next proceeds to act 9014, where it is determinedif there are any rows remaining in the Data Array. If there are no rowsremaining, the process continues to act 9020. Otherwise, if there arerows remaining the process continues to act 9016, where it is determinedif there is more than one row remaining in the Data Array. If there isonly one row remaining in the Data Array, then process continues to act9018 where the remaining row is removed from the Data Array and added tothe Solution Array. After the row is added to the Solution Array, theprocess continues to Act 9020. If there is more than one row remainingin the Data Array at act 9016, the process returns to Act 9010, wherethe row comparisons are repeated, this time using the new first row andcomparing the subsequent rows to the new first row. The process thencontinues again to act 9012 where the first row of the Data Array andany rows that were excluded in act 9010 are removed from the Data Array,and the resulting solution row for the removed rows is added to theSolution Array. For example, because Data Array 9044 in FIG. 24D hasmore than one remaining row, the second row will be compared to thefirst row using the equation of Table 9. The second row passes thistest, and thus both rows of the Data Array may be removed and thesolution row resulting from these two rows may be added to the SolutionArray. The resulting Data Array 9044 and Solution Array 9046 are shownin FIG. 24E. Data Array 9044 is empty, because both rows have beenremoved. Solution Array 9046, now includes an additional row, whichrepresents the parameters of a dwell for the two emitters correspondingto the two rows just removed from the Data Array.

The process then continues to act 9014, where it is again determined ifany rows are left in the Data Array. In the example of FIG. 24E, no morerows are left in Data Array 9044, so the process would continue to act9020. However, if there was more than one row left in the Data Array,the process would return to act 9010 to again determine if any rowscould be excluded given the new values of RVT_(MIN) and DD_(MIN).

Once the process reaches act 9020, the Data Array is empty and theSolution Array includes possible solutions for the dwell parameters. Forexample, Solution Array 9046 includes two rows, indicating that twodwells may be used to cover the emitters. The first dwell has an MDT of1 ms, an EDT of 19 ms, and an RVT of 500 ms, while the second dwell hasan MDT of 2 ms, an EDT of 11 ms, and an RVT of 700 ms. However, asdiscussed above, it is possible in some situations that merging thesetwo dwells into a single dwell may result in a lower cost than runningtwo separate dwells. The process of FIG. 23 evaluates this possibilityand determines if dwells should be maintained as separate dwells or ifdwells should be merged. As will be discussed in greater detail belowwith respect to FIG. 23, if, after some rows in the Solution Array aremerged, more than one row remains in the Solution Array, it may bedesirable to return to FIG. 22 to determine if any of the rows left inthe Solution Array may be excluded in determining DD_(MAX) for thedwell.

Thus, at act 9020 of FIG. 22, the process continues to act 9022 of FIG.23, where it is determined if there is more than one row in the SolutionArray. If there is only row in the Solution Array, then the processcontinues to act 9024 Solution Array is returned as the solution,indicating that one dwell may be used to cover the emitters, havingdwell parameters defined by the one row in the Solution Array. If thereis more than one row in the Solution Array, then the process continuesto act 9026, where the index variable N is set to one. The variable Nindicates the row in the solution that is currently being evaluated. Theprocess next continues to act 9028, where it is determined if Row N andRow N+1 of the Solution Array should be merged. This determination maybe made for example, by calculating the cost of using separate dwellsfor Row N and Row N+1 using the equation of Table 11 and comparing thiscost to the cost of using a single “merged” dwell that covers both row Nand row N+1. The cost of using a single merged dwell for both rows N andN+1 may be calculated, for example, using the equation of Table 3. Thatis, the cost of a merged dwell would be the DD_(MAX) of rows N and N+1divided by the RVT_(MIN) of rows N and N+1.

If using separate dwells yields a lower cost than a merged dwell, therows are kept separate. In the example of Solution Array 9046 of FIG.24E, there are two rows in the Solution Array. The cost of using twoseparate dwells for these two rows is approximately 0.003, as shown inTable 13. The cost of using a single merged dwell is 0.004, as shown inTable 14. Because the cost of using separate dwells is less, the rowsare not merged.

TABLE 13${cost} = {\frac{2 + {1\left( {\frac{700}{500} - 1} \right)}}{700}\quad}$cost ≈ .003

TABLE 14 $\begin{matrix}{{cost} = \frac{2}{500}} \\{{cost} = {.004}}\end{matrix}\quad$

In the cost computation examples in Table 13 and Table 14, minimum dwellduration (MDT) was used for the sake of simplicity. As mentioned above,the actual dwell duration may in some situations be greater than theminimum dwell duration. It should be appreciated that in any costcomputation involving dwell duration an estimation of the actual dwellduration may be used, for example, using the equations of Table 7 andTable 8 to determine an estimate of the actual dwell duration. However,for the sake of simplicity in the examples included herein, MDT maysometimes be substituted for the estimate of actual dwell duration.

If it is determined that the rows should not be merged at act 9028, theprocess continues to act 9030, where it is determined if N+1 is the lastrow in the solution array. If N+1 is the last row in the Solution Arraythe process continues to act 9032 where it is determined if any rowshave been merged. If no rows have been merged, the process continues toact 9054 where the Solution Array is returned as the solution. Each rowrepresents a dwell and the parameters of the dwell that will be used.Thus, in the example of Solution Array 9046, no rows were merged becauseit was decided to maintain separate dwells. FIG. 24F shows the SolutionArray that will be returned as the solution. Solution Array has tworows, thus two dwells may be used. The first dwell has an MDT of 1, anEDT 19 and an RVT of 500, while the second dwell has an MDT of 2, an EDTof 11, and an RVT of 700.

However, if at act 9032, it is determined that two or more rows havebeen previously merged (e.g., on a previous iteration) the processcontinues to act 9042, where all rows in the Solution Array are removedfrom the Solution Array and added back into the Data Array. The order ofthe rows in the Solution Array is maintained in the Data Array. Theprocess then returns back to act 9010 of FIG. 22, so that it may bedetermined, for the new rows created by the merging of rows, if it ispossible to exclude the dwell duration of any of these rows indetermining DD_(MAX). The process then continues from 9010 as describedabove, until the Data Array is again empty and the unique solutions fromthe Data Array have been moved in to the Solution Array.

However, if at act 9030, it is determined that more than one row remainsin the Solution Array the process continues to act 9034, where the indexvariable N is incremented by one. After the index variable N isincremented, the process returns to act 9028, where the determination asto whether rows should merged or kept separate is made again, this timewith respect to the new row N and row N+1.

If it is determined at act 9028 that rows N and N+1 should be merged,then the parameters of Row N are disregarded as a solution, and theparameters of Row N+1 are redefined as the DD_(MAX) and RVT_(MIN) ofRows N and N+1. Thus, pretending for the sake of illustration, that rows1 and 2 in FIG. 24F yield a lower cost as a single merged dwell(although, as discussed above, these rows actually yield a lower cost asseparate dwells), then row 1 would be disregarded and row 2 would beupdates so that it's parameters are the DD_(MAX) and RVT_(MIN) of row 1and row 2 (i.e., MDT of 2, EDT of 19, and RVT of 500). The process thencontinues to act 9038, where it is determined if row N+1 is the last rowin the Solution Array. If Row N+1 is not the last row in the SolutionArray then the process continues to act 9034, where the value of theindex variable N is incremented by one, and the process returns to act9028 to evaluate whether the next two rows in the Solution Array shouldbe merged or kept separate. If Row N+1 is the last row in the SolutionArray, the process continues to act 9040, where it is determined if thenumber of rows in the Solution Array is greater than one.

If there is only one row in the Solution Array, the process continues toact 9054, where the Solution Array is returned as the solution. That is,one dwell will be used with the parameters of the row in the SolutionArray. If there is more than one row in the Solution Array, the processcontinues to act 9042, where all rows are removed from the SolutionArray into the Data Array and the process returns to act 9010 of FIG.22. As discussed above, at act 9010 of FIG. 22 the “new” rows created bymerging may be determined if it is possible to exclude the dwellduration of any of these rows in determining DD_(MAX).

In this manner, it may be determined for a proposed dwell generated whenextracting the scan strategy, if the proposed minimum dwell duration maybe decreased, and if more than one dwell covering separate emitterswould be more cost effective. It should be appreciated that this methodmay be performed each time dwell cost is computed or used only sometimeswhen dwell cost is computed.

Tuning Step Coverage Gap Avoidance

Once the scan strategy is created, a post-processing check may beperformed to ensure that no coverage gaps have been introduced betweenadjacent dwells, as a result of rounding down the minimum frequencybetween dwells. When the scan strategy is created, if the minimumfrequency of a dwell is not an integer multiple of the tuning step ofthe receiver, then the frequency range of the dwell will be shifted downso that the minimum frequency of the dwell is an integer multiple of thetuning step size. For example suppose the tuning step of the receiver is10 MHz and a dwell is created having a frequency range of 1255 MHz-1355MHz. When the scan strategy is created, the frequency range of the dwellmay be rounded down to 1250 MHz-1350 MHz, so that the minimum frequencyof the dwell (i.e., 1250 MHz) is an integer multiple of the tuning stepsize (i.e., 10 MHz).

For example, FIG. 25 shows a portion of a scan strategy 9070. Theportion of scan strategy 9070 includes adjacent dwells 1-12. A dwell isadjacent to another dwell if the frequency range of that dwell endswhere the frequency range of the other dwell begins or if the frequencyrange of that dwell begins where the frequency range of the other dwellends. For example, in the portion of scan strategy 9070, dwells 1 and 2are adjacent to each other because the frequency range of dwell 1 endswhere the frequency range of dwell 2 begins. Similarly, dwells 2 and 3are adjacent, dwell 3 and 4 are adjacent, etc. Hence, dwells 1-12 are ablock of adjacent dwells. In the example of FIG. 25, each of the dwells1-12 has the same bandwidth. However, it should be appreciated that thedwells do not necessarily have to have the same bandwidth to be a blockof adjacent dwells. Dwells 1-12 together cover Emitters 1, 2, and 3.

Scan strategy 9071 shows the portion of scan strategy 9070 after thefrequency range of each dwell has been rounded down so that the minimumfrequency of each of the dwells 1-12 is an integer multiple of tuningstep size. As a result of this rounding down, there is a portion of thefrequency spectrum 9072 that is no longer covered by any of the dwells1-12. This portion of the frequency spectrum was previously covered bydwell 12 in scan strategy 9070, but was left uncovered when thefrequency range of dwell 12 was rounded down in scan strategy 9071.Nonetheless, there is no coverage gap introduced because the entirefrequency range of Emitter 3 is still covered by dwells 9, 10, 11, and12. However, if the frequency range of Emitter 3 had extended into theportion of the frequency spectrum 9072, then a portion of the frequencyrange in which Emitter 3 may operate is left uncovered by the scanstrategy.

For example, in FIG. 26 scan strategy 9073 is a portion of scan strategyagain including 12 adjacent dwells (i.e., dwells 1-12). These dwellstogether are intended to cover Emitters 4, 5, and 6. Scan strategy 9074results after the dwells of scan strategy 9073 are rounded down so thattheir minimum frequencies are integer multiples of the tuning step size.Similar to FIG. 25, a portion of the frequency spectrum 9075 that waspreviously covered by dwell 12 is left uncovered after dwell 12 isrounded down. However, in this instance, the frequency range in whichEmitter 6 may operate extends into this portion of the frequencyspectrum 9075. Thus, because a portion of the frequency range of Emitter6 is not covered by a dwell, a coverage gap is introduced.

In one embodiment of the invention, this problem is detected andcorrected by providing an addition dwell to cover the portion of thefrequency spectrum left uncovered by the rounding down of dwells.Typically, coverage gaps are only problem for smaller bandwidth dwells.In the case of larger bandwidth dwells, when the receiver is steppedthrough the frequency range covered by the dwell and reaches the top ofthe frequency range of the dwell (i.e., the last tuning step of thedwell), the bandwidth of the detecting method is wide enough to pick upany emitter signals that are left uncovered above the maximum frequencyrange of the dwell. Therefore, in one embodiment of the invention, onlythe smallest bandwidth dwells are checked to determine if any coveragegaps have been introduced as a result of rounding down the minimumfrequency of dwells. However, it should be appreciated that all dwellsmay be checked for this problem or only a subset of dwells may bechecked.

FIG. 27 is a flowchart illustrating a method for determining if anycoverage gaps exist as a result of rounding down the smallest bandwidthdwells of the scan strategy. At act 9080 a Data Array is created. TheData Array is an array based on the Information Matrix. Each row in theData Array represents an emitter/mode for which the scan strategy wascreated to detect. The columns in the Data Array represent theparameters of these emitters (e.g., RF Min, RF Max, dwell duration,revisit times, etc.). After the Data Array is created, the processcontinues to Act 9081 where the Scan Array is created based on the scanstrategy. Each row in the Scan Array represents a dwell in the scanstrategy. The columns of the Scan Array represent the parameters of thedwell (e.g., MDT, EDT, RVT, etc.). Once the Scan Array is created, theprocess continues to act 9082, where the Scan Array is used to identifyblocks of adjacent smallest bandwidth dwells. A block may include one ormore dwells. Also at act 9082, the index variable N is set to one. Thisindex variable is used to identify which block identified in act 9082 iscurrently being analyzed.

The process next continues to act 9083 where any emitters that aredetectable at the top of block N are identified. That is, any emittersthat are detectable by the top most dwell (i.e., highest maximumfrequency dwell) in the block are identified. This includes emitterswhose maximum frequency exceeds the maximum frequency of the dwell by atmost one tuning step. The process then continues to act 9084 where it isdetermined if the highest frequencies the emitters identified in act9083 are below the maximum frequency of the top most dwell in block N.If there are no emitters whose maximum frequency exceeds the maximumfrequency of the block, then the process continues to act 9086, where itis determined if all blocks have been evaluated. If all blocks have beenevaluated for the existence of coverage gaps then the process ends atact 9089. Otherwise, if some blocks have not yet been evaluated theprocess continues to act 9088 where the value of N is incremented by oneand the process returns to act 9083 and the next block of adjacentdwells is evaluated for the existence of coverage gaps.

If, at act 9084, it is determined that one or more emitters' highestfrequency exceeds the highest frequency of the top most dwell of blockN, then the process continues to act 9085. At act 9085 it is determinedif the uncovered portion of the frequency spectrum created by therounding down of the top most dwell of block N is covered by anotherdwell and if the emitter or emitters which are uncovered are detectableby that dwell. This may be determined, for example, by searching theScan Array for additional dwells which cover that portion of thefrequency range, whose parameters are suitable for detecting theuncovered emitter or emitters.

If the uncovered portion of the emitter or emitters is detectable byanother dwell, then the process continues to act 9086 where it isdetermined if all blocks have been evaluated. If all blocks have beenevaluated for the existence of coverage gaps then the process ends atact 9089. Otherwise, if some blocks have not yet been evaluated theprocess continues to act 9088 where the value of N is incremented by oneand the process returns to act 9083 and the next block of adjacentdwells is evaluated for the existence of coverage gaps.

Otherwise, if the uncovered portion of the emitter or emitters is notdetectable by another dwell, a new dwell is created and this additionaldwell is appended to the top most dwell of block N. The parameters ofthis dwell are set so that they cover the emitter or emitters that wereleft uncovered as a result of the rounding down of the top most dwell ofblock N. That is, the dwell duration and revisit time of the additionaldwell will be the DD_(MAX) and RVT_(MIN) of the emitter or emitters thatwere left uncovered.

After the additional dwell has been created, the process continues toact 9086 where it is determined if all blocks have been evaluated. Ifall blocks have been evaluated for the existence of coverage gaps thenthe process ends at act 9089. Otherwise, if some blocks have not yetbeen evaluated the process continues to act 9088 where the value of N isincremented by one and the process returns to act 9083 and the nextblock of adjacent dwells is evaluated for the existence of coveragegaps.

It should be appreciated that if it is known that all dwells have aminimum frequency that is an integer multiple of the tuning step size,then it may not be necessary to check for coverage gaps. Manymodifications to the general algorithm shown in FIG. 27 may occur tothose skilled in the art and these are intended to be within the spiritand scope of the invention.

Resource Verification and Allocation

As described above, a scan strategy may be generated using data from theinformation matrix. However, the scan strategy may not be realizable dueto hardware or software limitations of the receiver system. In oneembodiment of the invention, the scan strategy is checked to verify thatthe scan strategy is realizable by the receiver system. If it isdetermined that the scan strategy is not realizable, then the scanstrategy may be replaced or modified to fit within the capacity of thereceiver system.

The limitations of the receiver system may be expressed as a limit onthe total number of dwells and limits on the quantity of dwell types.That is, the receiver system may have capacity for a certain number ofdwells total, as well as capacity for a certain number of dwells foreach unique instantaneous frequency (IF) of the detecting methods. Analgorithm may be used to check for dwell capacity violations. If anycapacity violations are found, the excess capacity may be removed fromthe scan strategy. The portion of the scan strategy that is removed maybe replaced with a scan strategy that is constrained not to use theconsumed receiver system assets.

An algorithm for determining if a scan strategy is realizable by thereceiver system and for constraining the scan strategy to fit within thecapacity of the receiver system according to one embodiment of theinvention is described below. This algorithm may be performed as apost-processing task, after the scan strategy has been generated. Thealgorithm tests the scan strategy from the widest IF bandwidth to thesmallest. As capacity limits are reached for a particular IF bandwidth,the excess capacity dwells are removed, so that the capacity issatisfied for that particular bandwidth. These dwells may be replaced,for example, by using the dwell placement algorithms shown in FIGS. 15,16, and 18, to create new dwells that cover the frequency range of theremoved dwells. However, when the dwell placement algorithm is used, itwill be constrained to only using detecting methods having an IFbandwidth less than that of the removed dwells. This process may berepeated for each IF bandwidth, from widest to smallest. As a result,capacity violations are “bow waved” to smaller IF bandwidths.

If the smallest IF bandwidth is checked and capacity violations stillexist, then dwells may be discarded from the scan strategy. However,because any capacity violations have been “bow waved” to the smallest IFbandwidth, any discarded dwells will use a detecting method of thesmallest IF bandwidth. Thus, the amount of frequency coverage that islost by discarding dwells is reduced because the discarded dwells aresmallest IF bandwidth dwells. Additionally, if dwells are discarded, anerror may be logged which indicates to the operator that some frequencycoverage may have been lost.

FIG. 28 and FIG. 29 are flow charts illustrating an example of such analgorithm. At act 5100 of FIG. 28, DATA, SCAN TABLE and CAPACITIES arereceived. DATA is a matrix of emitter parameters, where each row in thematrix represents an emitter that is covered by the scan strategy. DATAmay be, for example, the information matrix used in creating the scanstrategy. SCAN TABLE is a table describing the scan strategy. That is,SCAN TABLE is a table of the dwells in the scan strategy. CAPACITIESincludes information relating to the capacity of the receiver system.The capacity of the receiver system may be expressed in various ways.For example, the capacity may be expressed in limitations on number ofdwells permitted and number of dwells permitted per unique IF bandwidth,and number of hardware calibrations permitted for a dwell. While theexamples below discuss receiver system capacity in terms of calibrationsit should be appreciated that any suitable measure at capacity may beused and the invention is not limited in this respect.

Once DATA, SCAN TABLE, and CAPACITIES are received, the processcontinues to act 5101, where an IF Array is created and the indexvariable, n, is set to one. The IF Array may be, for example, aone-dimensional array, that lists all the unique IF bandwidths, orderedfrom the widest IF bandwidth to the smallest IF bandwidth. As discussedabove, a detecting method is a particular IF/VBW bandwidth combination.Thus, for example, if there are eight unique detecting methodsavailable, there may be, for example, four unique IF bandwidths. In thiscase, the IF Array would include these four unique bandwidths, orderedfrom widest to smallest. In the example information matrix shown in FIG.13, there are two detecting methods available. The first has an IFbandwidth of 250 MHz and the second has an IF bandwidth of 30 MHz. Thus,if only these two detecting methods were available, the IF Array wouldinclude these two elements. 250 MHz would precede 30 MHz in the IF Arraybecause an IF bandwidth of 250 MHz is wider than an IF bandwidth of 30MHz. It should be appreciated that, as any number of detecting methodsmay be used, the IF Array may include any number of elements, as theinvention is not limited in this respect. The index variable n indicatesthe position in the IF Array that is currently being examined. Thus, atact 5101 n is initialized to one.

The process continues to act 5102 where the number of calibrationsneeded for the dwells in the scan strategy having an IF bandwidth of thenth element of the IF Array is determined. A calibration refers to thehardware configuration of a dwell. Some dwells may only differ in termsof timing (i.e., dwell duration and revisit time). If these dwells sharethe same hardware configuration, then they share the same calibration.Thus, there may be a certain number of unique dwells of a particular IFbandwidth, but the number of unique hardware configurations for thosedwells may be less than that number (i.e., if two or more dwells havethe same hardware configuration). Thus, two dwells have a the samecalibration if they have the same IF bandwidth, video bandwidth (VBW),and center frequency. Typically, more receiver system resources arerequired to support the wider bandwidth dwells. Receiver systems areusually calibrated versus frequency. Thus, the wider the IF bandwidth ofthe dwell, the more frequency calibration points are required.

Once the number of frequency calibrations that are needed for the dwellsof that particular IF bandwidth is determined the process continues toact 5103, where it is determined if the number of calibrations for theparticular IF bandwidth determined at act 5102 exceeds the number ofallowable calibrations for the particular IF bandwidth. Thisdetermination may be made, for example, by using the information inCAPACITIES.

If the number of frequency calibrations needed for the dwells of theparticular IF bandwidth exceed the limitation imposed by the receiversystem, the process continues to act 5104. Here, the variable X isdefined as the number of dwells that exceed the capacity to bediscarded. That is, at act 5104, the number of dwells that may bediscarded so that the number of frequency calibrations needed meets thelimitation of the receiver system is determined. For the sake ofclarity, this number is represented by the variable X. The process thencontinues to act 5105 where the X dwells with the largest revisit timesare identified. These dwells are the dwells to be discarded.

The process continues to act 5106 where the subset of emitter in DATAthat the X dwells were are intended to intercept is determined. Thus,the emitters (or portions of emitters) that are left uncovered bydiscarding the X dwells is determined. The process then continues to act5107, where a new scan strategy is created for the subset of emittersthat were left uncovered. This may be accomplished, for example, bycreating an information matrix using the subset of emitters in DATA thatare left uncovered, and performing the methods illustrated in FIG. 15and FIG. 16 for determining a scan strategy, using this informationmatrix. However, the method of FIG. 16 is constrained so that detectingmethods having an IF bandwidth greater than or equal to that of thediscarded dwells cannot be used. That is, these detecting methods may beexcluded from the Method Vector. Thus, a new scan strategy results forthis subset of emitters, and this new scan strategy may be appended tothe existing scan strategy, to provide coverage for the subset ofemitters that were left uncovered by the discarding the X dwells.

Once the new scan strategy has been created and appended to the existingscan strategy the process continues to act 5108. If, at act 5103, it wasdetermined that the number of calibrations does not exceed thecalibration capacity of the receiver for that particular IF bandwidth,the process goes directly to act 5108. At act 5108, the total number ofdwells with an IF bandwidth of the current IF bandwidth being analyzed(i.e., the nth element of the IF Array) is determined. The process thencontinues to act 5109 where it is determined if the number of dwellsdetermined in 5108 exceed the receiver's capacity for dwells of theparticular IF bandwidth. This determination may be made, for example, bycomparing the number of dwells determined in act 5108 to the limit ondwells of the particular IF bandwidth specified in CAPACITIES.

If the total number of dwells for the IF bandwidth being examinedexceeds the receiver system capacity, the process continues to act 5111.At act 5111 any independent dwells of the particular IF bandwidth areidentified. These dwells are merged, starting with the independentdwells having the largest revisit times, until the number of dwellsfalls within the receiver system limits. By merging the dwells withlargest revisit times first, the cost savings that is lost by using amerged dwell instead of separate dwells is reduced. The process thencontinues to act 5110 where it is determined if the nth element of theIF Array is the last element of the IF Array. If it is, then all IFbandwidths have been examined and the process continues to act 5112. Ifnot all IF bandwidths have been examined then the process continues toact 5113, where the index variable n is incremented by one and theprocess repeats for the next IF bandwidth in the IF Array. That is, thenext IF bandwidth in the IF Array will be examined to determine if thenumber of calibrations needed for that particular IF bandwidth fitswithin the receiver system capacity (i.e., acts 5102-5107) and if thenumber of dwells of that IF bandwidth fits within the receiver systemcapacity (i.e., acts 5108-5111). Thus, each IF bandwidth in the IF Arraymay be examined.

As mentioned above, once the last element in the IF array has beenexamined, as described above, the process continues to act 5112. At act5112, the process continues to act 5113 of FIG. 29. At act 5122 of FIG.29, the total size of the scan strategy is determined. The total size ofthe scan strategy may be expressed as the amount of receiver systemmemory needed to store and support the scan strategy. The process thencontinues to act 5114 where it is determined if the total size of thescan strategy is within the receiver system's capacity. If the scanstrategy is within the capacity of the receiver system then the processends at act 5121.

Otherwise, the process continues to act 5115 where the dwell of thesmallest IF bandwidth are identified. The process continues to act 5116where any independent dwells of the smallest IF bandwidth are mergeduntil the capacity limit is satisfied or all independent dwells aremerged. The process then continues to act 5117 where the size of scanstrategy is again evaluated to determine if it is within the receiversystem's capacity. If the scan strategy size is within the receiversystem capacity, then the process continues to act 5121 where theprocess ends. Otherwise, the process continues to act 5118 where thenumber of dwells to be discarded so that the scan strategy size iswithin the receiver system capacity is determined. This number isdefined as Z. The process then continues to act 5119 where Z smallest IFbandwidth dwells are discarded. The process then continues to act 5120where an error message is logged for the operator indicating that asubset of emitters is left uncovered by the Z dwells which werediscarded. The process then continues to act 5121 where the processends.

It should be appreciated that many modifications may be made to thegeneral algorithm described above and these are intended to be withinthe spirit and scope of the invention.

Intercept Performance Evaluation

Once a scan strategy is created, the scan strategy's interceptperformance against a given emitter set under specific altitude, rangeand receiver load conditions may be evaluated. Evaluating the scanstrategy may include modeling 2D and 3D emitter scan patterns, and thecompilation of performance statistics for each emitter mode analyzed.

Intercept performance evaluation allows for an independent means ofvalidating the scan strategy. A scan strategy may be built by applyingsets of algorithms to determine the best way to satisfy a group ofemitter characteristics. If the algorithms are perfect, and the emitterdata does not conflict, then the scan strategy should be correct.However, the possible combinations of input data and their interactionscan be very complex, so an independent means of validating the scanstrategy may be desirable. As a result, one avoids the cost of labtesting with actual receiver system hardware and software to proveprobability of intercept and mean time to intercept performance. Thus,each emitter is tested for detectability and intercept performanceagainst the probability of detection and mean time to interceptrequirements and reports are produced for a user to review.

Intercept performance evaluation may also take into account performancevariation as a result of scenario assumptions. The scan strategy may bebuilt for a specific set of emitters under specific engagementscenarios. Intercept performance evaluation allows variation in thescenario assumptions from those for which the scan strategy was built,and allows for evaluation of how well a scan strategy built for one setof emitters might perform for a different set.

An algorithm for intercept performance is described below. The algorithmuses a DATA matrix, which is a matrix of emitter parameters. Each row inDATA represents an emitter/mode. The algorithm also uses an EmitterList, that includes a list of emitters in DATA selected for probabilityof intercept and mean time to intercept analysis. The algorithm also auses the scan strategy that was built. The scan strategy includes a setof dwells whose performance will be evaluated against the emitters inthe Emitter List. Lastly, the algorithm uses a set of scenarioassumptions. These scenario assumptions are parameters which define theintercept conditions and may include altitude, velocity, and scan loadestimate.

The algorithm begins by computing the probability of intercept and meantime to intercept for each entry in the Emitter List. This may beaccomplished by, for each emitter in the emitter list, finding thatemitter in the DATA matrix and extracting its parameters. Then, thesubset of dwells in scan strategy that provide coverage based onhardware attributes of the dwell (e.g., frequency coverage, pulse widthcoverage) is identified. If no dwell is found in the scan strategy, acoverage error may be logged to the user and the next emitter in theEmitter List. If one or more dwells is identified, the emitter'sfrequency range may be broken down into discrete pieces, as defined bythe intersection with subset of detection dwell. FIG. 30 illustrates thebreakdown of an emitter's frequency range into discrete pieces. Asillustrated in FIG. 30, each portion of the emitter's frequency rangethat is covered by a unique combination of dwells is considered adiscrete piece.

For each discrete frequency piece, p, identify the subset of dwells thatcover that frequency piece. For each dwell of the subset of dwells, thefollowing steps are performed. A simulation may be performed for eachdwell against the emitter to generate a set of time-to-interceptresults. The simulation for generating the time-to-intercept resultswill be discussed below in greater detail. Next, a normalized histogramof the time-to-intercept data may be created. Additional plots of thedata may also be created. Next, the time-to-intercept data may beconverted to probability of detection and mean time to interceptstatistics. The probability of detection may be determined based on thepercentage of time-to-intercept times less than or equal to requiredintercept time. The mean time to intercept may be generated based on theaverage of all time-to-intercept values. Further, the statisticsgenerated by the simulation for each dwell and for each frequency piecemay be stored.

Next, for each discrete frequency piece, p, the contribution of thecomposite dwells, to that frequency piece may be computed. Table 15shows an equation for computing the contribution of the composite dwells(n) to the mean time to intercept (MTTI) for p. Table 16 shows anequation for computing the contribution of the composite dwells (n) tothe probability of detection (Pd) for p.

TABLE 15 ${MTTI}_{p} = {\sum\limits_{n}\;\frac{1}{{MTTI}_{n}}}$

TABLE 16${Pd}_{p} = {1 - {\prod\limits_{n}\;\left( {1 - {Pd}_{n}} \right)}}$

Using the equations of Table 15 and Table 16, the minimum performancemay be recorded. The minimum performance is represented by the smallestPd_(p) value and largest MTTI_(p) value. Also, the overall performancemay be determined. The overall Pd and the overall MTTI are the weightedaverage of Pd_(p) and MTTI_(p) respectively. The weight for each pieceis the ratio of each piece's frequency range divided by the emitter'sfrequency range.

As mentioned above, a simulation may be run to generate a set oftime-to-intercept values. In one embodiment of the invention, a MonteCarlo simulation is performed to generate the data used to determine thestatistics of intercept of a specific dwell against a specific emitter.The emitter's scan is simulated and time above dwell sensitivity isnoted, reduced by the amount of time required for data collection. Thisrepresents the illumination time periods available for intercept. FIG.31 shows an emitter scan pattern. FIG. 31 also shows an sensitivitythreshold, based on the sensitivity of the dwell. The dwell's time ofexecution is modeled as a random process with a mean value of revisittime. “Jitter” about the mean may be represented by a constant tosimulate process noise. The mean revisit time may also be scaled tosimulate receiver loading (i.e., utilization). The start time of theemitter's scan and the start time of the first dwell are both randomizedat the start of each Monte Carlo trial. The trial ends when the dwellstart time falls within one of the time periods available for interceptand the dwell duration captures at least one pulse or signal sample ofthe emitter. The time from emitter scan start to this time is recordedas the Time-to-Intercept for this trial. The number of Monte Carlotrials to perform may be selected by a user or may be specified inanother manner. An array of time-to-intercept values is returned.

An algorithm for performing the simulation is described below. Thealgorithm uses a set of Emitter Parameters of the emitter to evaluate, aset of dwell parameters of a dwell for which the emitter will beevaluated, and a set of scenario assumptions, including parameters whichdefine the intercept conditions (e.g., altitude, velocity, and scan loadestimate, utilization)

The algorithm begins by using the set of scenario assumptions todetermine Antenna/Scan pattern detectability. If there is real antennadata available, this data may be used in determining the antennadetectability. Otherwise, a propagation model may be used. If theemitter is a steady emitter, the propagation module may useinsignificant amplitude modulation as a function of time/angle. If theemitter is a 2D emitter (e.g., sector, circular), the propagation modelmay use amplitude modulation as a function of time/angle in the azimuthplane only. If the emitter is a 3D emitter (e.g., electronic, mechanicalraster), the propagation module may use amplitude modulation as afunction of time/angle in several elevation planes and across azimuth.

If the maximum amplitude is less than the dwell's sensitivity, then zeromay be recorded as the time-to-intercept to indicate that the emitter isnot detectable by the dwell. Otherwise, the start and stop times of eachillumination time per scan period may be recorded. As shown in FIG. 32,the stop time may be reduce by the desired minimum integration time todeclare a detection (e.g., several pulses or samples).

Once the antenna scan pattern is determined, a series of Monte Carlotrials may be performed. For each Monte Carlo trial, the following actsmay be performed. First antenna pointing angle (antenna scan start) israndomized. The revisit time (RVT) may then be scaled by the load factor(i.e., RVT=RVT×Utilization). Next, the initial time T (i.e., time forfirst dwell execution) is set as a random value of process noise jitter.Next it is determined if T intersects one of the illumination periods ofthe scan pattern. If T does not intersect one of the illuminations, thenT may be incremented by RVT plus process noise jitter (jitteralways<RVT) and again it may be determined if T intersects one of theillumination periods. If T grows too large, the time-to-intercept forthis Monte Carlo trial is recorded as “infinity” and the next trial maybe run. If T does intersect one of the illumination periods, it may bedetermined if the dwell duration (T, T+MDT) brackets at least one pulseor sample (i.e., if the receiver dwelled long enough to declaredetection).

If the dwell duration did not bracket at least one pulse, T may beincreased by RVT plus process noise jitter and it may again bedetermined if T intersects one of the illumination periods, as describedabove. Otherwise, T may be recorded as a valid time-to-intercept valuefor this Monte Carlo trial and the algorithm may proceed to next trial.

Having described several embodiments of the invention in detail, variousmodifications and improvements will readily occur to those skilled inthe art. Such modifications and improvements are intended to be withinthe spirit and scope of the invention. Accordingly, the foregoingdescription is by way of example only, and is not intended as limiting.The invention is limited only as defined by the following claims andequivalents thereto.

1. A method of detecting an emitter signal, comprising acts of: a)receiving a scan strategy to detect at least one emitter, the scanstrategy comprising a plurality of dwells; b) simulating a scan of anemitter; and c) evaluating the performance of at least one of theplurality of dwells of the scan strategy based on the scan of theemitter.
 2. The method of claim 1, wherein the act b) further comprisesan act of: simulating the scan of an emitter using real antenna data. 3.The method of claim 1, wherein the act b) further comprises an act of:simulating the scan of an emitter using a propagation model.
 4. Themethod of claim 1, wherein the act c) further comprises an act of:dividing a frequency range of the emitter into a plurality of discretefrequency pieces, each respective frequency piece being defined by aunique combination of dwells from the plurality of dwells that cover afrequency range of the respective frequency piece.
 5. The method ofclaim 1, further comprising acts of: for at least one frequency piece ofthe plurality of frequency pieces, determining the dwells of the uniquecombination of dwells that cover the frequency piece; and generating aset of time-to-intercept values that for a dwell of the uniquecombination of dwells against the emitter.
 6. The method of claim 5,wherein the act of generating the time-to-intercept values furthercomprises: performing a series of simulations to determine the set oftime-to-intercept values for the dwell against the emitter, wherein aninitial time of execution of the dwell with respect to the scan of theemitter is varied in at least some of the trials.
 7. The method of claim6, wherein the act of performing a series of simulations furthercomprises factoring in scenario assumptions into the simulation.
 8. Themethod of claim 7, wherein the scenario simulations include at least oneof altitude, velocity, and utilization.
 9. The method of claim 5,further comprising an act of: generating probability of interceptstatistics and mean time to intercept statistics based on thetime-to-intercept values.
 10. A computer-readable medium, having encodedthereon computer instructions which when executed by a computer systemcause the computer system to perform a method comprising acts of: a)receiving a scan strategy to detect at least one emitter, the scanstrategy comprising a plurality of dwells; b) simulating a scan of anemitter; and c) evaluating the performance of at least one of theplurality of dwells of the scan strategy based on the scan of theemitter.
 11. The computer-readable medium of claim 10, wherein the actb) further comprises an act of: simulating the scan of an emitter usingreal antenna data.
 12. The computer-readable medium of claim 10, whereinthe act b) further comprises an act of: simulating the scan of anemitter using a propagation model.
 13. The computer-readable medium ofclaim 10, wherein the act c) further comprises an act of: dividing afrequency range of the emitter into a plurality of discrete frequencypieces, each respective frequency piece being defined by a uniquecombination of dwells from the plurality of dwells that cover afrequency range of the respective frequency piece.
 14. Thecomputer-readable medium of claim 13, wherein the method furthercomprises acts of: for at least one frequency piece of the plurality offrequency pieces, determining the dwells of the unique combination ofdwells that cover the frequency piece; and generating a set oftime-to-intercept values that for a dwell of the unique combination ofdwells against the emitter.
 15. The computer-readable medium of claim14, wherein the act of generating the time-to-intercept values furthercomprises: performing a series of simulations to determine the set oftime-to-intercept values for the dwell against the emitter, wherein aninitial time of execution of the dwell with respect to the scan of theemitter is varied in at least some of the trials.
 16. Thecomputer-readable medium of claim 15, wherein the act of performing aseries of simulations further comprises factoring in scenarioassumptions into the simulation.
 17. The computer-readable medium ofclaim 16, wherein the scenario simulations include at least one ofaltitude, velocity, and utilization.
 18. A receiver system for detectingan emitter signal comprising: a memory having stored therein a scanstrategy, the scan strategy having been evaluated by a system thatperforms acts of: a) receiving a scan strategy to detect at least oneemitter, the scan strategy comprising a plurality of dwells; b)simulating a scan of an emitter; and c) evaluating the performance of atleast one of the plurality of dwells of the scan strategy based on thescan of the emitter.
 19. The receiver system of claim 18, wherein theact c) further comprises an act of: dividing a frequency range of theemitter into a plurality of discrete frequency pieces, each respectivefrequency piece being defined by a unique combination of dwells from theplurality of dwells that cover a frequency range of the respectivefrequency piece.
 20. The receiver system of claim 19, wherein the systemfurther performs acts of: for at least one frequency piece of theplurality of frequency pieces, determining the dwells of the uniquecombination of dwells that cover the frequency piece; and generating aset of time-to-intercept values that for a dwell of the uniquecombination of dwells against the emitter.